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Going Loco

BLOG - A closer look at our collection of historic locomotives

With a collection of locomotives dating from Victorian times to the 1960s, there's plenty to discover.


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Deeper Underground?* Part 2

So, we found out about the origins of western end of Crossrail last time but we didn't get to the raw meat. Again, with assistance from Harry Pettit, let's take a second look at the G.W.R. Womble Trains.**

A train of white-painted MICAs headed for Smithfield and approaching Paddington station in the 1930s.

The passenger services weren't the only part of the GWR's involvement with things under London. There was a freight service that required the special attentions of the both the Carriage & Wagon and the Locomotive Department. This was the service that linked Acton freight yard to the Smithfield Meat Market.

A plan of the GWR depot at Smithfield.

MICA van 105860 at Didcot Railway Centre in the 1980s.

This service lead to the development of the MICA range of 4 wheeled refrigerated freight vans. Originally, fresh meat was transported in ordinary ventilated vans with the obvious drawback of ventilation not being the same as refrigeration. In order to preserve the product better, in 1897 the first of the refrigerated vans with their characteristic ice boxes on each end and white paint scheme with red lettering was built in the X diagram.



The MICA van is in the carriage shed for a complete overhaul with new timber purchased by owners, the 813 Fund. The project will re-commence when volunteers are allowed back on site after the current lockdown.

We have one of these vehicles, a diagram X8, owned by our friends in the 813 Fund. No. 105860 was built towards the end of the MICA building period in 1925 and is currently being restored with all new woodwork by our Carriage & Wagon Department. We also have the fabulous model of TOAD No. 56985 in 7¼ in scale in the museum. If you look at this Diagram AA7 vehicle, you will see that it is much shorter than the standard TOAD designs. This is to facilitate it working the confined space of the Smithfield Branch.

The 7¼ in scale model of the Toad in the Museum and Archive at Didcot.

The issue the services presented for the Locomotive Department - as it was with all underground steam workings - was the tendency of steam engines to produce a lot of steam and smoke. No, really, they do! This tends to fill up tunnels pretty fast and make the environment basically a bit anti-breathing.

The solution to this is to have a valve in the smokebox that can direct the exhaust from the chimney and put it in a container. In a steam engine, the obvious container is the water tank and this is kind of a good news, bad news situation. The good news is that not only do we get far less steam and smoke filling the tunnel, we also get a lot of the steam condensed back into water thereby extending the range of the locomotive. The bad news is that it makes the water hot and if you read my previous post on injectors, you will find that they generally don't work well on hot water. The solution is to have one Giffard type injector and one steam powered pump. The steam pump is less efficient but will work when the water gets hot. You can then use the more efficient Giffard injector when not in the tunnels.

633 class 0-6-0T No 642, built in 1872, fitted with condensing apparatus.

633 class 0-6-0T No 643, built in 1872, photographed by Dr Ian C Allen at Clapham Junction in October 1929. This locomotive retained the condensing apparatus for the rest of its life, being withdrawn in February 1934 having run more than a million miles.

The first of these ‘condensing’ locos built by the GWR. were two of the 633 Class 0-6-0 tank engines. These were standard gauge and the first was built in 1871. At first, only Nos. 643 and 644 were fitted with condensing gear but as time went on a number of them were retro fitted. The 633 Class were also known as ‘Get Wets’, due to having no cab roof...

No 9701 built in September 1933, one of the 11 condensing locomotives derived from the 57XX class 0-6-0 pannier tanks. This photograph shows the water feed pump mounted on the side of the smokebox.

In 1933, a condensing version of the ubiquitous 57XX 0-6-0 pannier tank locomotive was developed to replace the 633s. Nos. 9700 was used as the test bed and consequently Nos. 9701 to 9710 were built as condensers from new. No. 9700 was originally No. 8700 and had the old style cab, with porthole style windows but the others in the ‘class’ had the later style taller cab with large windows which is the same as Didcot's resident panniers, Nos. 3650 & 3738. They were easily identified as their ‘pannier’ tanks partly stretched down to the running plate (really making them side tanks) and the smokebox was exposed. On the driver's side of the smokebox was the water pump and there was a conglomeration of pipes over the top of the boiler and tanks which fed the exhaust smoke and steam into the water tanks. Sadly, none were preserved but they lasted until the end of the steam era. Just two classes in all that time goes to prove that they don't build ‘em like they used to...

Condensing locomotive No 9702.

The G.W.R. locos used on the shared underground lines also threw up a few issues. Firstly, the signalling system on the underground uses pieces of equipment called trip cocks. These are small valves under the locomotive that are opened by a piece of equipment at the side of the track if the train goes through a signal at danger. The valve lets the air out or in - depending upon if you have a vacuum or air based brake system - and stops the train. It was simple job to fit these to an existing steam loco. Another more serious problem was the conductor rails. The A.T.C. (Automatic Train Control) shoe mounted under the engines sits in the middle of the loco with the shoe dangling down. On the G.W.R. lines, this made contact with the ramps near the signals*** Unfortunately, this is where the underground lines saw fit to place one of their conductor rails. This has -210v DC running through it. When connected to the positive outside conductor rail (at +420v DC) it gives a combined voltage of 630v DC. Enough to really ruin your day if mishandled.

Condensing locomotive No 9703, photographed in 1962 by Mike Peart and still displaying GWR initials.

The locos that used the L.T. tracks were fitted with self-raising A.T.C. shoes. The sole surviving 61XX class loco, our very own No. 6106 is still fitted with all this gear as it was used on the passenger services described last time. As a result the access to the platforms that were equipped with the conductor rails was restricted to those classes fitted with the retractable A.T.C. gear. It didn't mean to say however that the wrong sort of engines didn't get sent to platforms 13 to 16. It is said that a King class locomotive was sent there in error once. The results were quite literally and figuratively shocking (!) as the centre rail was directly connected to earth through the loco frames, wheels and track. In a flash, a fairly large chunk of the L.T. system was shut down.

One imagines that there was some fairly extensive explaining to do after that...

A train from Smithfield emerging into the open air at Paddington behind one of the 9700 class locomotives.

*With sincere apologies to Jamiroquai this time round.

**Underground, Overground - just like the theme music. You lot know what I'm on about. Sing along now...

***See my blog on the A.T.C. system for more details.


Going Underground, Going Underground...* Part 1

A drawing of the original stations on the Metropolitan Railway

And now for something completely different.** My right hand man on the Pendennis Castle Project, Harry Pettit, has a thing for trains that disappear down into holes in the Earth as well as an entirely understandable love of Castle class 4-6-0s. He's helped me craft a two part blog on the strange link between the G.W.R., the London Underground and raw meat. Read on if you dare...


A trial trip on the Metropolitan Railway at Edgware Road, 1862. And A drawing of the broad gauge Metropolitan class locomotives built in 1862.

The Metropolitan Railway was the world's first passenger carrying underground railway. The company came into being in November 1852 and construction commenced in 1860. Now, the strange thing about this from modern eyes at least is the fact that these lines were built as dual gauge. When first built, the GWR was built to broad gauge. This means that Brunel decided to space the tracks further apart (at 7 ft 0¼ in than what we today regard as standard gauge (which is 4 ft 8½ in).*** This was done so that both the G.W.R. and the G.N.R. (Great Northern Railway which was built to standard gauge) could both run on the line which ran from Paddington to Farringdon.

A painting of a broad gauge train at Praed Street junction heading towards Paddington station. The line in the foreground is the westward extension towards Notting Hill Gate and Kensington.

Metropolitan class 2-4-0 No 3562 built in February 1894, fitted with large water tanks and condensing apparatus. After Metropolitan Railway electric locomotives took over the city services from Paddington in 1906 the condensing apparatus was removed and No 3562 worked on until she was withdrawn in 1949.

On the 10th January 1863 the Metropolitan Railway opened using GWR broad gauge coaches hauled by Metropolitan Class tank locomotives. 22 of these locos were completed between June 1862 and October 1864, and having the staying power of a mayfly, were withdrawn between June 1871 and December 1877. Soon after however, operational disagreements caused the GWR stock to be withdrawn from the Met in August 1863. From then, standard gauge trains ran through to Moorgate Street. These were extended to Bishopsgate (Liverpool Street) in 1875 and Aldgate in 1894. Some G.W.R. engines continued to operate on other parts of what is now the underground - the famous standard gauge ‘Metro Class’ 2-4-0 tank engines began service in 1869 and ran services on what was to become the Circle line until its electrification in 1906.

So how does this link with our collection at Didcot? While the G.W.R. no longer operated on the underground, their coaches still did and the trains would arrive at Paddington’s Platform 15 or 16 to be connected to a Metropolitan loco and hauled off to trundle around on the underground. The earlier 4 and 6 wheel coaches used on this service were eventually replaced in the 1920s with the C sets. These were coaches designed to a slightly smaller than regular profile to fit through the tunnels. These became known as the ‘Main Line & City’ coaches and were arranged into sets that went as follows:

Brake Third**** - All Third - Composite***** - Composite - All Third - Brake Third.

There were 9 full sets of coaches - 8 for service and a spare. Most were built in 1920 and 1921, but two sets built in 1925 were articulated. This is where two coach bodies are coupled together permanently with just one 4 wheeled bogie underneath. The idea here being that it reduced rolling resistance. Less wheels there are, the easier it is to move. The idea is still around today - just look at many of the high speed trains in use on the continent like the French T.G.V. This was less useful when one coach had a mechanical problem. This meant that the whole set had to be removed from service until it was fixed where as a spare coach could be substituted in a normally coupled set. As a result, only one set had all 6 coach bodies fully articulated. There was then an attempt to mitigate the problem by building 4 half sets of 3 coaches. Swindon kind of lost interest in articulation after that.

A Metropolitan Railway electric locomotive and rolling stock, showing the collector shoes under the brake van.

By the time these coaches appeared, the Metropolitan electric engines (like the famous preserved machine Sarah Siddons) had replaced steam on the Metropolitan services out of Paddington. This meant that the brake third coaches stock had electrical collector shoes on them that could pick up electricity from the two conductor rails on the underground. These were linked together and could be coupled to the electric locomotive. This prevented something called gapping. This is where an electric locomotive stops in a position where its collector shoes are not touching the conductor rails. This means no electricity, the engine can’t move and much embarrassment and delay occurs. Collector shoes at the other end of the train prevent this happening.

Southall station looking east in the 1908 to 1920 period. In the bay platform is a train of four-wheeled coaches and Metropolitan 2-4-0 tank engine No 1420, about to set off towards Paddington and the City. The first coach has the distinctive half-circle ventilators on the doors, a feature of reduced-height vehicles designed to go through the Metropolitan Railway tunnels. No 1420 was built in October 1878 and withdrawn in April 1936.

In 1937 the through service stopped and the Hammersmith branch became the Hammersmith and City line. The ‘City Service’ via the Metropolitan Railway ceased in 1939 with the outbreak of WWII. This left the coaches at a loose end. They went on to less and less important duties. The two survivors - brake thirds No. 3755 & 3756 - found their way into preservation at Didcot via being press ganged into service on the Glyncorrwg Branch where they were used on Miners’ trains to transfer personnel to and from the pit. They were stripped out internally and fitted with propane powered lighting. Making them among the last gas lit coaches in service in the U.K.


No 3755, one of the Main Line & City vehicles which replaced the 4-wheelers on the through services to Moorgate in the early 1920s. 3755 displays the Moorgate destination blind and is painted in the pseudo panelling livery with the steel panels lined out to match the mouldings of earlier wooden bodied coaches. The coach is 100 years old on 19 June this year. Crossrail, or Elizabeth Line, is due to start trial running later this year, but 3755 can proudly claim to have been taking commuters from the western London suburbs through to the City a century earlier!

They were finally withdrawn in 1965 and both Nos. 3755 and 3756 were purchased for preservation. They were initially delivered to Bridgnorth on the Severn Valley Railway and later moved to Didcot. No. 3756 is safely stored in essentially the same condition it was preserved in, awaiting restoration in the Carriage & Wagon Shed. No. 3755 however is fully restored and operational. Its non-corridor design with individual passenger compartments has been of great use in the Covid-19 situation. Social distancing train rides! When you think about it, these coaches and the service they were built for are very relevant to modern London. While their day ended in the 1930s, the idea was a good one - why do you think they have built Crossrail?

Paddington suburban station being rebuilt in September 1933 to enable the new 61XX class 2-6-2T locomotives to use the platforms. On the left is platform 16, used by eastbound Hammersmith and City line underground trains. This track goes under the GWR Goods Offices, with the corner of that building supported by girders to allow the excavation. Platform 15 has a 61XX class loco standing next to it. Platform 14 is still at an early stage of rebuilding. On the right the electrified platform 13 was used by westbound electric trains. When the works were complete the GWR steam trains normally used platforms 14 and 15, but one electric train each day went through those platforms to keep the conductor rails clean. In 1967 the track approaches to Paddington station were remodelled and the physical connection between Western Region and London Underground tracks was removed. Now platforms 15 and 16 are exclusively for Underground trains and 13 and 14 for GWR trains.

We will deal with the raw meat next time.

3755 displaying a full complement of oil tail lamps.

*With sincere apologies to The Jam...

**...and Monty Python.

***I’ve put a blog on this in the list of stuff we need to talk about.

****A coach with third class seating and a compartment for the guard & Luggage.

*****A coach equipped with both first and third class seating.


Actually, Totally Cool!*

2988 Rob Roy, showing the ATC shoe apparatus beneath the front buffer beam.

One of the things that the GWR prided itself on was a signalling system that was known as Automatic Train Control, or A.T.C. This was a quite remarkable feat of engineering in the predigital age. So, as you read this, remember that it was designed not that long after the 19th Century became the 20th. The system itself has to work in a manner that is known as ‘fail to safe’ or if something goes wrong, then everything stops. The system has two main parts on the engine. There is a box on the driver's side of the cab, most easily distinguished by the big brass bell on the top of it. This has a number of connections going in and out of it. More of which later...

The cab apparatus in No 6106 and the A.T.C. shoe fitted to No 7202 - this is easily visible because it is on the front of the loco, while most of the others at Didcot have theirs under the cab

6106's cab equipment is probably a unique survivor as it is the version with the shoe in or out indicator, for when the loco was running over the electrified lines into platforms 13 to 16 at Paddington. The 61XX and 97XX locos were fitted with these because of their regular trips over the electrified lines. The shoe was automatically lifted on the approach to the electrified lines, and lowered when they left them. The indication on the cab apparatus was a confirmation to the driver that it had taken place.

The other bit is more difficult to see and will be a second box with a metal shoe on a post sticking out of the bottom of it. This can be mounted in a range of different places, the most common being either at the front of the loco or under the cab. The early Castles for example had their shoe gear under the cab while the later ones had it at the front as built**. It's all the same equipment however and it does the same job. It just has to have the shoe mounted centrally between the rails and with the bottom of the shoe 2 ½” from the tops of the rails.

The third bit of this system isn’t attached to the engine at all and is in fact on the track, between the rails near the signal. This was a 44 foot long rail-like ramp that was made electrically live (had a current running through it) when the signal it was ‘attached’ to was clear. If the signal was at danger, it was electrically dead (no current). This is where the first fail to safe bit comes in. If the system is broken in any way - the ATC ramp for a signal isn’t working - it’s electrically dead, switched off. The system will therefore read it as at danger, no matter what the signal says. Failsafe No. 1.

So what does the two loco boxes do? Well, there is an electrical link between the two, via a battery that energises a pair of electromagnets in the box with the bell on it. These hold shut a valve that is linked to the vacuum brake system. The vacuum system also has a siren attached too. All the while the locomotive is going along and there are no signals, the battery keeps the valve shut. No battery voltage, the valve is open, the brakes cannot create a vacuum and the train is going nowhere. Failsafe No. 2!

Failsafe No. 3 is on the track side. There are two switches that both need to be turned on together in order for the ramp to be energised. There is a switch on the signal arm itself. Now, you may or may not know that G.W.R. Signals show they are clear by moving down. They are known as lower quadrant signals. The rear of the signal arm is a large metal casting so, in theory, it should counterbalance back to horizontal even if the wire snaps but there is the small chance that snow, ice, a fallen tree branch, or some other rogue element could force the end of the signal down, giving a false clear. As a result, the second switch was in the signal box and was only switched on by moving the lever for that signal to the clear position.


The ramp on the main demonstration line at Didcot - the ramp was originally on the main line at Didcot railway station.

A.T.C. ramp in everyday use. Southall station with 2-8-0 No 4708 hauling a goods train, which includes a crane, on the up relief line towards London. On the adjoining track, the down relief, the ATC ramp is visible. Photo taken by Mike Peart about 1962

When approaching a signal, the ramp lifts the shoe in the box under the engine by 1”. This operates a switch that disconnects the on-board battery from the system. This leaves the electromagnets holding the vacuum valve shut at the mercy of the ramp. If the signal is clear, then the electricity is supplied by the ramp and the current continues to flow to the electromagnets. The other thing this does is ring the bell. This gives the driver an audible confirmation that the road ahead is clear. If the shoe is lifted with the signal at danger, then there is no electricity going to the electromagnets and electromagnets without the electro bit aren’t very magnetic... This opens the vacuum system to the atmosphere. As the air rushes in to fill the vacuum, it goes through a siren and this gives an audible warning just before the brakes come on and stop the train.

On the side of the box in the cab is a little handle and this enables the driver to override the A.T.C. system if he needs to. It may sound a bit strange to want to override a perfectly good safety system but conceivably, there might be a situation whereby the train needs to stop but also needs to be removed from a position of danger. This handle enables this to happen. Now, as if this electro-mechanical system isn’t amazing enough, how about this? The system automatically turns itself off to save the battery charge! There is a small, vacuum operated switch on the circuit that is only turned on when there is a vacuum present in the system. No vacuum, the engine isn’t moving and therefore no need for the A.T.C. system to be on. Switch the system off and there is no current being drawn. Genius!

The really amazing thing about this is that as long as you position your ramps in the right place - far enough away from the relevant signals to allow a train at full line speed to stop - the driver can trust the system implicitly. And they did. They were won over by how simple and reliable it was. As a result, trains could keep moving at normal speeds where the system was fitted even in poor visibility.

This could prove terrifying for those not in the know. An old engineman's yarn*** went along the lines that a passenger train once had to be double headed**** with an L.N.E.R. (London & North Eastern Railway) locomotive behind a G.W.R. locomotive running on an A.T.C. equipped line. The majority of the L.N.E.R. did not have a comparable system and as the train plunged forth through dense fog at line speed, the crew of the L.N.E.R. loco became more and more terrified by the seemingly suicidal actions of the leading crew. Growing more and more nervous, the L.N.E.R. crew shut off steam and opened their brake valve, bringing the whole train to a juddering halt. The driver of the ‘foreign’ engine was incensed at the situation and went to the cab of the leading engine, ready to get some really good coarse to industrial grade Anglo Saxon flowing. He was put in his place however, when he was reminded by the G.W.R. driver (none too gently one imagines!) about what the A.T.C. system was and how well it worked...

True story or not, G.W.R. crews were right to be confident. Despite this being built firmly before the electronics age, this system kept G.W.R. travellers safe for decades. There was never an accident that was caused by the mechanical or electrical failure of the A.T.C. system. Later systems eventually replaced A.T.C. after the end of the steam era but it should still be admired as one of the unsung heroes of the rails. Its rugged simplicity and incredible ingenuity kept part of our nation moving quickly and safely. When we are able to reopen the Centre, you can discover more about the A.T.C. system in The Signalling Centre.

*Actually Totally Cool? A.T.C.? Get it? No? I’ll fetch my coat...

**For extra nerd points, on the Castles you can see where the shoe is by looking for an electrical conduit pipe running along the driver’s side running plate. No. 5051 has it to contain the wires that connect the shoe box to the cab box. No. 4079 doesn't as her shoe box was under the cab, so no need for wires going front to back!

***No verification, but it's a good tale!

****Two locos on the front of a train.

Dean Goods 0-6-0 No 2351 showing the ATC shoe under the front buffer beam 

FRIDAY 12 February

Working This Out Was A Real (Tractive) Effort!

The P2 class 2-8-2s had a relatively short life in their original form, being rebuilt as 4-6-2s by Gresley’s successor, Edward Thompson, in 1943. This reduced their tractive effort to 40,320lbf, still slightly ahead of the GWR’s Kings. They were classed A2/2. Thompson’s successor, A H Peppercorn, produced a development of the A2/2 class, the A2s introduced in 1947. They had a tractive effort of 40,430lbf which gave them the title of Britain's most powerful express passenger locomotives for the remainder of the steam era. This photograph is A2 No 60538 Velocity

So, I’ve frightened some in the past with a few impromptu science lessons, it’s about to get worse for some. Maths ahoy! It's not all that bad really - it’s one of those fantastic mathematical equations that is what might be referred to today as plug and play. You feed the numbers in and you get the information out that you wanted! So, what's this all about?


Southern Railway Lord Nelson class No 857 Lord Howe, a member of the Lord Nelson class which took the tractive effort prize from the GWR, at 33,510lbf when introduced in 1926

You will often find that in many publications and websites, that the power of a steam locomotive is described with a strange figure which is known as ‘Tractive Effort’ or T.E. What is stranger is that it is usually presented as a measurement in lb or imperial pounds or more accurately lbf which is pound of force. We will work in imperial measures here if that’s ok - much easier with the old school beasts we work with! Basically, it’s the pushing or pulling force that a locomotive can exert. It is WAY more complicated than that - there are such things as starting tractive effort, maximum tractive effort, continuous tractive effort and so on but as always we will simplify this down to a more layman’s perspective.

The Erie Railroad in the USA employed and “triplex” locomotives including 2-8-8-8-2 Matt H Shay (built 1914) with the driving force spread over three sets of wheels. The locomotive was reputed to be capable of hauling 650 freight cars!

This isn’t a fool proof measure of the strength of a locomotive. This gives you the fact that it can certainly start a train with close to this tractive effort. If you take the example of the steam locomotive that had the highest tractive effort in history, the picture becomes less clear. This was a 6 cylinder 2-8-8-8-2 monster that had a tractive effort of 199,560lbf! Great you all say! 50 coach passenger trains, here we come! Well, no. The issue was that the cylinders of the ‘Triplex’ were so big that it could empty all the steam out of the boiler if you were to open the regulator all the way and go past 5mph. So that amazing tractive effort was practically useless. Tractive effort is a reasonable measure of a locomotive but there are a lot of other factors to consider when we think about how practical a steam locomotive is.

4073 Caerphilly Castle, which claimed to be the most powerful express passenger locomotive in Britain with a tractive effort of 31,625lbf when introduced in 1923. This photograph was taken during 4073s stay at Didcot in the 1990s, between leaving the Science Museum in London and being installed in the Steam Museum at Swindon

As you might know from our Pendennis Castle updates, there was a bit of a controversy about tractive effort that ended in a set of trials in 1925 involving No. 4079 herself. I won’t spoil that further here. This talk of tractive effort of passenger locomotives became something of a P.R. point of honour and the Castles (31,625lbf.) were beaten by the Southern Railway in 1926 when they introduced the Lord Nelson class in 1926 with a tractive effort of 33,510lbf. The GWR decided fight back and introduce the King Class at 40,285lbf. in 1927.


6000 King George V flying the flag for Britain at the Baltimore & Ohio Railroad's Fair of the Iron Horse in 1927. The King class regained the most powerful express passenger locomotive crown for the GWR with 40,285lbf but interestingly TE wasn't amongst the dimensions listed on this postcard of the era


The Princess Royal class 4-6-2s, designed by ex-Swindon man William Stanier for the LMS, equalled the tractive effort of the GWR’s King class when introduced in 1933, having identical cylinder dimensions, boiler pressure and driving wheel diameter. This photograph by R A Panting shows 46201 Princess Elizabeth ready to depart from Euston on 30 May 1953. The locomotive is now preserved

This settled the deal for a while until the LMS built the Princess  Royal class 4-6-2s in 1933 with identical dimensions to the King class of cylinder diameter and stroke, boiler pressure and driving wheel diameter. Therefore identical tractive effort of 40,285lbf. Quickly after that, the LNER built the P2 Class 2-8-2s which topped out at 43,462lbf. These locos were rebuilt into less than successful 4-6-2s during WWII but the new build P2 No. 2007 Prince of Wales will return this class to reality and slightly increase its tractive effort to 43,684lbf. For U.K. steam at least it will probably remain a record for a long time to come.

The LNER’s P2 class 2-8-2s, designed by Nigel Gresley, took the accolade of Britain’s most powerful express passenger locomotives when introduced in 1934, with tractive effort of 43,462lbf. This photograph was taken of No 2001 Cock O’The North at New Barnet by Charles Gordon Stuart in 1934. Charles left a generous bequest to the Great Western Trust which enabled the Charles Gordon Stuart annexe to the museum and archive at Didcot Railway Centre to be built

The engineers that designed any steam engine will have given the locomotive several characteristics that will determine its eventual tractive effort. The variables we are going to work with to give us a very reasonable approximation of a given loco's tractive effort. These are as follows:

t = the tractive effort in pounds of force or lbf.

What we want to know!

d = The diameter of the piston in inches (“).

The bigger the area of the piston, the more area the pressure of the steam has to push on.

s = the stroke (distance the piston travels in the cylinder in either direction) in inches (“).

The longer distance that the piston has to travel then the more the steam can expand. This allows more of its energy to be exerted on the piston before it’s exhausted up the chimney.

p = the working pressure of the boiler in pounds per square inch (psi).

The higher the psi of the boiler, the more energy the steam has in it when it gets to the cylinders.

w = the diameter of the driving wheels in inches (“).

If a loco has large wheels it can get up to higher speeds - one revolution of the wheels moves the engine a long way forward - but it pays a price for doing so. A large wheel is difficult to get turning. A smaller wheel had less resistance to being turned and more of the energy of the engine can go to pulling the train. It will however limit your top speed because the pistons can only go backwards and forwards so fast before things begin to break! This is why express passenger engines have big wheels, mixed traffic (passenger and freight) engines have middle sized wheels and freight and shunting engines have small wheels. Their wheel size is ‘tuned’ to their job.

The Coronation class 4-6-2s introduced by the LMS in 1937 were an enlargement of the Princess Royal class, but with their larger 6ft 9in diameter driving wheels their tractive effort was lower, at 40,000lbf exactly. This photograph, by F R Hebron, shows 46240 City of Coventry enjoying the attention of a large cleaning gang

We also use something called a constant. This is to do with the efficiency. No matter how good we are, forces such as friction, losses in heat energy as the steam moves around and so on will always bite into the numbers. If we didn’t use a constant, we would get the theoretical maximum output of the engine but there is no way it could ever achieve it. You can try to disobey a lot of laws but the ones set down by physics are pretty tough to break... American and British engineers used a constant of 0.85 so, when we do the last bit of the equation we get 85% of the total theoretical output. Here's how to do it for a two cylinder locomotive.

Hmmmm - let’s translate that into bite size chunks shall we?

Step 1: multiply the diameter of the piston (d) by itself.

Step 2: multiply the piston stroke (s) by the working pressure of the boiler (p).

Step 3: multiply together your answers from Step 1 and Step 2.

Step 4: divide your answer from Step 3 by the wheel diameter (w).

Step 5: multiply your answer to Step 4 by 0.85

Step 6: If your engine has 2 cylinders, go directly to Step 7. If it has 3 cylinders multiply your answer to Step 5 by 1.5 and if it has 4 cylinders, multiply your answer to Step 5 by 2. It is rare for steam locos to have more than 4 cylinders in the U.K.

Step 7: round your answer to something reasonable if you need to, then pat yourself on your back and look smug. You just calculated your first tractive effort!

As with all maths lessons, let’s do a worked example using No. 2999 Lady of Legend as our example. The things we need to know about her are:

d = 18.5” diameter pistons**

s = 30” piston stroke

p = 225 psi boiler pressure

w = 80.5” diameter driving wheels*

The Saint Class is a 2 cylinder design.

It will help you to write it all down as I have above. So the tractive effort of Lady of Legend is:

Step 1: 18.5 x 18.5 = 342.25

Step 2: 30 x 225 = 6,750

Step 3: 342.25 x 6,750 = 2,310,187.5

Step 4: 2,310,187.5

80.5. = 28,697.98136645963

Step 5: 28,697.98136645963 x 0.85 = 24,393.28416149068lbf.

Step 6: Ignore this step as a Saint Class has 2 cylinders. Go directly to Step 7.

Step 7: let’s round that to a more easily digested 24,393lbf. The published figure for a Saint** is usually quoted as about 24,395lbf. You may engage smug mode.***

The great thing about this is that you can pick up the required data for doing this calculation (and the published answer so you can see if you got it right!) on line for the vast majority of steam locomotives the world over. It is interesting to run the figures and see exactly why a King has a higher tractive effort than a Castle or why a Hall has a lower tractive effort than a 28XX 2-8-0, despite having the same size boiler. It’s a great little glimpse into the world of steam locomotive designers.

Well, what are you waiting for? Off you go and try it out...

Tell us how you got on!


*You might need to convert this figure from feet and inches to inches. For non steam mechanics, non Americans or those just not used to the old ways, there are 12 inches in one foot. A Saint has 6’ 8 1/2” driving wheels. So, 12 x 6 is 72. Then add the 8 1/2” and you get 80.5”. Remember to convert your fractions to decimals!

**The early saints had 18” cylinders, the later ones had 18.5” cylinders. Our one has the older outside style but the later inside diameter. Make sure you look up the right figure!

***One for the Red Dwarf fans out there - you know who you are...



Wonderful Winter Wagons Part 4 - It’s Go Go on the MOGO but No Go on the ASMO*

So, in the fourth and final chunk of wagon goodness for now we will take a look at the covered road vehicle wagons we have in the fleet. These are offshoots of the standard 4 wheeled covered van design but with a few evolutionary adaptations to their specialised purpose. The GWR was transporting road vehicles ever since its inception and the carriage of (horse) carriages was commonplace.

A royal luggage train at Acton on 21 June 1897, hauled by No 3040 'Empress of India' - note the horse-drawn carriage amongst the items being conveyed.

The later specially built vehicles all lived under the same index letter - G. The open wagons (which we have 2 different examples of**) really deserve a blog of their own but suffice to say that the types included HYDRAs, LORIOTs, SERPENTs and, MAYFLYs. Ironically, the MAYFLYs were converted from old brake vans in 1919 and were all scrapped by 1940. Talk about living up to your name...

There were also the BOCARs. These beasts were truly freight carrying in that their sole job was to move completed car bodies from one factory to another. These operated between Oxford and the Midlands and were sort of an outlier being based on long flat bogie trucks originally but later using converted coach chassis. These were provided with numerous tie down points to prevent the car bodies moving and getting damaged and then a framework that supported a large canvas cover that went over the top.

The reason for the specialist covered vans was due to the increasing road vehicle ownership in the U.K. In 1920 there were about 20,000 cars in private hands. In 1915 there were just 101,000 lorries and vans. By the 1930s that had become 1,000,000 cars and 350,000 commercial vehicles. While this rise in ownership was detrimental to the business of carrying passengers and freight, the GWR were smart enough to realise that moving the vehicles themselves were a potential source of revenue.

A train of ASMOs loaded with cars at Morris Cowley, headed for the Scottish Motor Show in Glasgow in 1930, with Aberdare class 2-6-0 No 2653 in charge

The first of these covered road vehicle vans was introduced in 1925 and eventually ran to three main types. In ascending size these were the MOGO, DAMO and ASMO. The sizes were as follows:

MOGO = 20’ 6” (6.111 metres) long

DAMO B = 23’ (7.01 metres) long

DAMO A = 32’ 5” (9.881 metres) long

ASMO = 36’ 4” (11.074 metres) long. 

The ASMO train at Morris Cowley loaded for the Scottish Motor Show at Glasgow in 1938, with Dean Goods No 2383 in charge

They all were built to essentially the same 4 wheel design. They had an inside frame body with a small pair of double doors in each side. The interior was completely open and had a series of tie down points and chocks on the floor to secure the vehicle. Both ends have a pair of large doors and a fold down section that forms a ‘bridge’ over the buffers. The wagon could be pushed up to a loading ramp and road vehicles driven straight in or out. The wagons were fitted with vacuum braking systems and were capable of running in both fast freight and passenger trains.

The GWS ASMO at Abingdon station on 23 May 1970 when the GWS had a weekend display there. Somehow the team managed to fit three Austin 7s (owned by Kevin McCormack, GWS secretary at the time) in the ASMO, although it is only designed to take two cars!

The one failing with the design was that in most cases, there were just the two doors on each end of the wagons. The doors were too big to open if the wagons were coupled together. This meant that each wagon could only be loaded or unloaded one at a time. If you had to load 10 wagons, a locomotive would have to move each of them to the loading ramp in turn. This was time consuming and labour intensive. In later versions the end doors were hinged in the middle, just like a modern bi-fold door. This meant that ALL the doors in the train could be opened at once while still coupled together and the road vehicles driven through the train without uncoupling them. Something still done on today’s car carrying wagons.


The GWS exhibition train returning from Wallingford Carnival on 20 June 1970. The ASMO is the fourth vehicle behind the two steam locos, 1466 and 6106. The train is hauled by D6343, one of the North British class 22 diesel-hydraulics. The head code OP03 suggests its day job is a station pilot at Paddington, hauling empty coaching stock to and from the station. As an aside, the class 22s were 50% longer and heavier than the 57xx pannier tanks they replaced on the station pilot duties, and the panniers had been cheaper to build and more reliable - but that's probably for another blog!

There are preserved examples of all three of these wagons. However, if you want to see a DAMO, you have to go to our friends at the South Devon Railway to see it. They have just (at the time of writing) begun the restoration of DAMO B No. 42223. This is the sole survivor of the type. MOGOs, are fairly common in preservation and our version was built in 1936 to diagram G.31. No. 105742 is fully restored and is capable of being used in our display freight trains. The sole surviving ASMO is also ours but it’s definitely on the to do list...

Our ASMO had a starring role in Thank You Comrades, a television play broadcast on BBC One on 19 December 1978. The play dealt with the film industry in the early days of the Soviet Union. A train was sent around the country to show propaganda films to the peasantry, with the ASMO taking on the role for the filming at Didcot

116954 was one of 80 similar vehicles built in 1930 to diagram G26. ASMOs would have been be a common sight around Didcot as they were used for both MG at Abingdon and Morris at Cowley. As it stands, it is unrestored, being stripped down with the wood panelling removed. It was started some time ago but other projects have sadly overtaken it. No doubt it will get done. It is currently not on public view, being safely contained in our carriage shed, awaiting its turn in the works.

Restored MOGO 105742 at Didcot

On the outside, the only clue to these vehicles’ fascinating history and their connection to the 40,000,000 cars that now inhabit our roads are the large end doors. If you didn’t know that then it’s just another van! It is remarkable however that despite the fact that a great deal of the freight that once went on the railways is now on the roads (and we are arguably worse off because of it...), cars are still being transported by rail and still go through the junction at Didcot. When you next visit, keep an eye out for the gigantic light blue articulated sets that occasionally trundle past on the main line. That’s one of the amazing things that Didcot does like almost nowhere else, while you look at our preserved vehicles, outside the fence their modern equivalents are ‘displayed’ as well. A historical juxtaposition like no other.

P.S. It would be interesting for your blogger to know if this little off topic diversion into the wagon fleet was interesting to our readers - please let us know if another dive into the wagon box would be appreciated! We have a load of coaches too...

* Alliteration for the nation?

**We have HYDRA D No. 42193 and LORIOT L No. 42271 but these are a bit too close to the CROCODILEs for inclusion this time round. They are on the blog list...



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