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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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A Tender Behind!

I know.

The title.

Couldn’t resist it.

So, tenders. There is always a whole bunch of stuff to be said about the locomotives that pull them but the tenders seem to get a bit of a raw deal. These fascinating vehicles have their own unique and varied history and deserve a little of the limelight as well. So, in the first of a two-parter, pull on the overalls and pick up the inspection lamp and we will go and have a look!*

The vast majority of early locomotives were tender engines. Simply because it’s more convenient to carry your coal and water supply on a separate vehicle rather than on the main chassis of the locomotive itself. Indeed, Trevithick’s early 19th Century prototypes were tender engines. The basics of these early vehicles are wonderfully illustrated by Stephenson’s Rocket. A simple wooden 4-wheeled truck with a flat area that you can put the fuel on and a large barrel, turned on it’s side to put the water in. Simpler times...

Replica Fire Fly in action on the Broad Gauge demonstration line

Looking forward to the earliest (albeit replica) versions in the Didcot collection, we come to the broad gauge versions. You can see the development between that of Rocket to Firefly and Iron Duke. The size of the tank is much larger and is of a riveted construction. The engines were consuming a lot more water as they became more powerful. They also were burning more fuel, hence the general increase in size. The basic shape is one that went through the whole of GWR tender development - in fact many of the design cues on Firefly can be traced through to almost the last GWR tenders built. Features that made it through include the rolled over top edge to the tank sides - although this was radically altered over the years - and the six axle configuration.

Features to note here include the sandwich type construction of the frames. This is an early railway technique where to ensure the strength of the materials, two plates of metal are sandwiched either side of a fairly serious lump of timber. The tank is a fair bit narrower than the frames with a lip around the outside. Some times this lip had the leaf springs above them and sometimes the springs were in / on the frames below. Some even had a small hutch type structure on them in which the train’s guard sat. It must have been exciting to ride there! Unbelievably this isn’t the most scary thing about the originals. That is that the handbrake on these vehicles were quite often the ONLY brake on the train.

Oh, and the brake blocks were wooden.

And on one side of the tender only...

Iron Duke class ‘Lightning’ showing the ‘iron coffin’ on back of the tender where a travelling porter sat looking backwards to watch the train for any signs of a mishap

The rivetted construction of the tender water tanks can be clearly seen on the replica Iron Duke; pictured here during a tender repaint.

We have a bit of a gap in the preservation story here as the oldest preserved GWR standard gauge tender locomotive** is the National Collection’s wonderful Dean Goods No. 2516. The 2,500 gallon tender preserved with this loco is noteworthy on our journey. It is a product of the William Dean era at Swindon and tenders of this era were often constructed with coal rails. These are exactly what they sound like - a metal cage structure that is used to hold the coal in! Later on these were replaced with the now familiar GWR style ‘flare’ section of flat steel plate and the short example shown on this vehicle is typical of the era. Again, many of the features from the broad gauge follow through including the tender tank being significantly narrower than the frames but times were a changing.

Early stage of construction of 2999’s tender tank. Note the well for extra water capacity between the wheels

Our next example comes in the form of the recreation of our Saint class Locomotive No. 2999 Lady of Legend. The Churchward era brought a whole new idea to Swindon - Standardisation. As far as was possible Mr Churchward tried to make as many components as interchangeable. Tenders are an obvious place to look. They are a separate vehicle attached to the locomotive after all! Tender No. 1719 was originally built in 1906 but oddly it’s frames were later replaced with a more modern set and the tank was rebuilt back to its original version for the Saint Project so its a bit of a mix. Probably not much there that dates back to 1906 any more but technically it is registered as having been built in the right time period so, we’ll take it! For serious nerd points, it’s the angles of the plate work where the axle boxes and springs are mounted that give the game away. Narrow plate work = early, wider plate work = late.

2999 and tender part way through (re)construction

The tank has been recreated to look as much like the original as possible and is a huge credit to Peter Gransden and Ali Matthews who built it. It might look like there are no rivets holding it together but there are. HUNDREDS of them. These early tenders were all flush riveted. The process is to drill a countersunk hole on the front. Insert your red hot rivet from behind the plate and then flatten the protruding stem into the countersink. Then you have to grind it flat to blend in with the sides. Imagine the hours of work in this job alone...

2999’s water tank with baffles fitted. Ali Matthews, who built it, standing in the tank

The tank now is almost as wide as the frames leaving just a small lip around the edge. A rolled top is in evidence as is the flare - in its original shorter Dean style but scaled up to fit the new larger 3,500 gallon design. To increase the water capacity, there is a well which protrudes down through the frames. The filler is in the centre, at the rear. The other important feature here is the step structure at the front of the tender. In order to make these tenders truly interchangeable with locomotives of different heights, there were different designs of this shovelling plate to suit specific classes.

2999's tender with the coal sheets fitted

Like the Dean Goods, No. 2999’s tender has its own vacuum brake system and the handbrake for the engine is on the left or fireman’s side of the cab. The other large handle is to operate the water scoop - a subject of a previous blog for you to discover! There are two smaller handles, either side of the coal space. These are the water valves and start the water flowing from the tender to the injectors to be pushed from there into the boiler. There is also a small vacuum valve here that can be turned to allow air into the tender brake system.

The finished locomotive - complete with the all-important tender

Well, that’s enough for now - in part two we will look at the tender designs of Mr Collett and Mr Hawksworth as well as a few of the oddities produced down the years. Like much of the engineering on our locos - they can talk to you and tell you a story if you know how to read it. Your next lesson in how to speak tender will be next week...

*Disclaimer: I KNOW that I am really skimming the surface here. Just pointing out the basics. There is a LOT more to it than this blog can or indeed should hold. If you need more, there is an excellent ongoing study here which really gets into the nitty - gritty.

**Some parts of the Bluebell Railway’s ‘Dukedog’ No. 9017 possibly come from earlier GWR tender locos but as this is only the cab and the boiler it’s not a pure survival like the Dean Goods. The tender is a later version too...


Hydrostatic Hyperbole?

It’s time we talked about hydrostatic displacement lubrication. Please don’t run away - this is really quite interesting! In fact this a fascinating piece of equipment that is vital to the operation of the majority of our steam locomotives but it is something that the average visitor never really realises the significance of. So, let’s warm up the spanners and take a dive into the works shall we?

Hydrostatic displacement lubrication relies on a very simple scientific principle that almost everyone knows. Oil floats on water. This is due to density. The measurement of density is its “mass per unit volume”. Do not be frightened by the sudden outbreak of physics. Let us explain. You may have heard of the old trick question:

“What is heavier - a ton of bricks or a ton of feathers?”

The answer is of course that they both weigh the same (1 ton! Who got that wrong?) but there will be a fundamental difference in the size of each of those loads. In our imaginary ton of bricks there will only be relatively few bricks compared to the probably tens of thousands of feathers needed to make the same weight, or more accurately, mass. A much bigger load. So there is a lot more of the stuff that makes up bricks in a brick than there is of the stuff that makes up feathers in a feather. We measure the amount of stuff a substance by measuring its mass (physics uses kilograms these days). We then define how much space the object is taking up. This is its volume (using centimetres cubed for solids and millilitres for liquids*). Divide the mass by the volume and you get the density of the mass per unit volume. See? It’s not so scary! So, for a millilitre of water there is more of the stuff that makes up water than the amount stuff that makes up oil in a millilitre of oil. That is why the oil floats on the water. It is less dense.

So what has this impromptu physics lesson got to do with steam engines and lubricators? When we last spoke about lubrication, we found out that there were two types of oil. The thinner general lubricating oil and the thicker steam oil. It’s the thicker stuff, that looks a bit like green golden syrup, that we need to deliver to the moving parts where the hostile steam filled environment exists. This is the regulator, valves and cylinders at the front of the engine. In the cab of an engine there is a big brass container of the stuff and it has pipes going into it and pipes coming out of it.

The pipes coming in do so via a strange thing, usually mounted on the cab roof that either looks like a heater bar or a ring off a cooker. They are not heaters - think about where the firebox is... They are in fact condensers. The steam from the boiler is turned back into water by having to travel through this long, winding and uninsulated pipe. It however, remains at boiler pressure. This water is fed into the big brass container and because oil floats on water, the water is at the bottom and the oil is on top. There are a number of galleries or passages that the oil is forced through when the steam pressure is applied and the amount of water in the container increases.

The galleries are also connected to the glass tubes at the front of the brass container. There can be as many as five glasses on GWR engines. Each glass tube has a tap near it and a cone type structure inside it. As the water comes into the container and pushes on the oil, some of the oil is forced up through these cones. As it is working, the oil will drip upwards through the sight glass. The tap controls how fast the drips go a though the cone and therefore how fast the oil is delivered to the front end.

You will also notice that the some of the steam pipes go through a valve that is connected to the regulator lever (the big red ‘accelerator’ control, via a mechanical linkage. This is called the jockey valve and means that the oil isn’t wasted as it only flows when the regulator is open. It does give us a problem though. What if we are going along but the steam is shut off - what we call ‘coasting’? We might coast for a long distance and that means no oil flows and that is not great for the valves and cylinders...

Well, when you move a regulator, you notice that there is a little bit of what feels like slack in the mechanism. The handle moves a bit and then sort of ‘bites’ and opens the regulator valve. When they want to coast, a GWR engine driver will shut the regulator lever right down but then they do a strange thing and lift it up again just slightly. This is enough movement to open the jockey valve but not open the regulator valve - clever stuff, right?!

When the pressure is on the oil it is forced from the lubricator to the front end. You can trace the pipes from the sight glasses to where they disappear under the cladding (the thin metal outer case that holds the insulation on the boiler). These pipes then travel under the lagging to keep things warm. The hotter this thick oil is, the easier it flows. These pipes come up from under the lagging at the front end of the boiler, goes through a series of shut off taps under a small cover** and then dive into the smokebox. These oil pipes then deliver the oil into the steam flow coming from the boiler. It passes through an atomiser and it is carried from there, in the steam, to the moving parts. The number of sight glasses was updated over the years and if you see a locomotive with just one cover on the driver’s side of the smokebox it has a 3 glass lubricator, if it has a second, smaller one on the fireman’s side them it has a 5 glass lubricator. Nerdy fact that one!***


So, if this system is so good, why don’t all of the GWR locomotives have a hydrostatic displacement lubricator? Well, this system is really great as it gives a very fine degree of control to the driver over the oil supply. For most of the first half of the 20th Century, when the GWR fitted superheaters to their engines they did so using just a low degree of superheat. There were a maximum of just two rows of superheater elements in the boiler and the lower resulting steam temperatures involved give you a bit more leeway in oil control. However, when the poor post war coal meant that some locos began to be fitted with three and even four row superheaters (like our own No. 6023 King Edward II), there was a growing concern that the drivers might not supply enough oil to match the increased temperatures that their steads now operated at. As a result, mechanical lubricators were fitted to the running plates of the engines and as these are driven by the locomotive mechanism as it moves, the rate of oil flow is taken out of the hands of the driver completely.

The drivers were not all behind this decision. It was quite common for a GWR driver, when faced with an engine that was running in a sluggish manner, to turn up the level of oil flow to the front end and this could result in a marked improvement in the running qualities of the machine. Indeed, when the first two post war Castle Class engines were built with three row superheaters, they were fitted with hydrostatic displacement lubricators. They were regarded as powerful and free running but when they and the subsequent post war Castles were fitted with mechanical lubrication, their reputation with some crews were not as good. The debate about the merits of both systems carry on to this day.

Wherever the truth lies, the hydrostatic displacement lubricator remains a both a singularly very clever and mostly unsung piece of locomotive engineering. So, the next time you are at Didcot, have a look and see it you can see one of these fine pieces of equipment. Ask our drivers about them and take a moment to marvel at it. It’s really quite amazing. Let’s hear it for the hydrostatic displacement lubricator!

PS: Your blogger was asked about his personal mass per unit volume while writing this. He declined to comment...

*You might remember from your science lessons that 1ml of a liquid is the same volume as 1 cubic centimetre. While we don’t use the metric system a great deal in the works, this is really handy for doing physics!

**For some unknown reason, the myth is that the little blister style covers for the oil pipes and valves are there to hide the superheater. No - a superheater is a massive thing. It bolts to the front of the boiler and fills a reasonable proportion of the smokebox and the boiler tubes. It does not need covering. It’s inside. Pre-covered. The little covers are just there to tidy up the look of oil pipes and valves. This is the second time we have mentioned this but this myth is really persistent. We are here to both entertain and educate!

***This argument is null and void when you have an engine with a mechanical lubricator. It’s a big box thing on the running plate with loads of pipes on - you can’t miss it!



The Return of the Champion - Chapter 1

It’s time to come clean. One of your blog writers here has another duty at Didcot Railway Centre. For a number of years, I have had the unbelievable privilege of looking after one of the Great Western Society’s most precious and famous residents. None other than No. 4079 Pendennis Castle. As you may or may not know, we repatriated the locomotive from Australia in 2000* and in 2001 we slowly took the engine to pieces, intending it to be restored to running order. Yours truly showed up a short time after and once I earned my laurels, I was given the job of looking after the engine. It’s been a long road, a great deal of work has been done already and there is light now at the end of the tunnel (pun intended). For the first of these semi - regular updates, we will take a look at the at some of what has been done already and what the latest updates are.

Photo taken on 7 August 2018

The whole engine was basically complete upon return but also had a few ‘extras’. There were a number of Australian fittings including electric lamp brackets, air brake fittings and most notably what seemed like half of the iron red sand of the Pilbara! It was only relatively recently that the last of this was ejected into the bin, discovered as a thin red haze on the last few untouched bits...

The engine has been right down to the frames. It has meant that we have peered into every corner of this mighty machine and it has needed a lot of work to get us to where we are today. The boiler - quite often the bete noir of many loco restoration project - was pretty good. No. 4079 was fed a diet of de-mineralised water upon her arrival in the Pilbara. Rio Tinto needed it to prevent the desert water from ruining their mining equipment. Also, when they finished running her, the Australian crew emptied her boiler out. The desert heat did the rest.

Inside Pistons

It required some work on the stays - the crown stays in particular - seam and foundation ring rivets and of course a complete re-tube. The frames and mechanical items were sadly a much different story. The structure of the engine and the tender under the cab was pretty much life expired. As this forms the drag box (the bit that connects the two vehicles together and transmits the pulling force of the loco to the train), we were keen that this was a good as possible. A total replacement of the metal here was undertaken with just the main frames and two complex (and thankfully reusable!) pressings remaining. Her front buffer beam was also twisted and a great deal of effort was expended in straightening this out too.

4079 Drag Box

There isn’t much mechanically that hasn’t been overhauled. The wear in the bearing surfaces in some cases was twice the scrapping size. Oil and red desert sand mixing together make terrible lubricant but amazing grinding paste... All the whitemetal bearings have been refreshed or renewed and the cylinder and valve liners are also new with pistons and valves machines to suit.

Valve Liner

A number of features required a bit of reversion back to her original specification. The injectors that No. 4079 went to Australia with don’t like hot water and as you can imagine, a tender tank of water in the desert gets pretty warm, pretty quickly. They were replaced with some injectors that were far less susceptible to this problem. We replaced them with two GWR live steam units. Not entirely authentic (she should have a live and an exhaust injector**) but more so than the Gresham and Craven units as supplied!

4079 Crank Axle

The other ‘start from scratch’ area was the cylinder drain cocks. These little valves are on the bottom of the cylinders and steam chest. These are opened as an engine stands still and remain open for the first couple of revolutions of the wheels to drain the water out of the cylinders. Hence the big whooosh when a steam engine moves off. Water is incompressible unlike steam and if you have too much of it in there your cylinders go bang in a really expensive way. There are 11 of these and they had all been replaced with steam operating versions from an unknown source before she left the U.K. in the 1970s. The GWR ones are operated via a single lever and a mechanical linkage. Now, casting, machining and grinding in these valves are all technical tasks. Getting all 11 to line up and open together took the patience requirement a stage further...

Cylinder Liners

We had the majority of the work on the frames and boiler completed and we reunited them in order to make sure that we had all the bits we needed. The loco had been in bits for a number of years and this we felt was important to ensure that there were no delays that might unnecessarily burn up vital boiler ticket time. A lot of very fiddly and unexciting but very vital jobs got sorted in this time. Just before the lockdown of 2020 started, we had decided to make the final push towards removing the boiler for completion and commissioning.

Connecting Rod

We had everything to the boiler cladding released and ready to come off. Then it’s just a case of the cab roof and boiler out! And that’s when lockdown hit.

For various medical reasons concerning my other half, I was promptly locked into my flat for the next 4 months...

Thankfully, my great friends in the loco works were able to come back to Didcot a lot sooner than I was. They took the cladding sheets and cab roof off and then lifted the boiler, placing it on BR WELTROL No. 901002. Work has now commenced on doing the last few jobs and making it fully pressure tight. I finally made it back to Didcot on the 1st August.

Team fitting Drain Cocks

Despite the fact that the workforce size has been limited, progress has been great. As soon as there is something more to report, I’ll write another blog with a little engine history, a few jobs we have done before and where we are at the moment.

Best Wishes,

Drew and the 4079 Team.

Photo taken 28 November 1970

*Sounds like yours truly has a few Castle history blogs to write here...

**I’ll do a blog post on the way injectors work - it’s REALLY clever!



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