The pannier tank locomotive as a type was first introduced by the GWR in 1898, on 4-4-0T No. 1490. It is easily recognised as a small engine somewhat resembling a matchbox, with its water tanks neither curving over the top of the boiler (saddle tank), nor standing on the running plate (side tanks). This arrangement allows easy access to the rodding etc. between the frames whilst not raising the centre of gravity as high as a saddle tank would.
Production of saddle tank locomotives, inherently unstable at speed, ceased on the GWR after 1910. Construction of the 57xx class, based on the 27xx class of 1901, began in 1929. In 1933, detail design improvements were made during the production of no 8750, thus introducing the sub-class 875x, of which 3738 is a member. Batches of numbers allocated within this very large class did not necessarily run consecutively.
3738 emerged from Swindon works in September 1937. She was allocated to Old Oak Common depot in London. After a working life of around half a million miles, she was sent for scrapping to Woodham Bros. of Barry in August 1965. From there she was bought by two Society members, who brought it to Didcot in 1974, where she was restored to full operational use in 1976. She then ran until the mid 1980s when she became due for a full overhaul. She joined the queue and was returned to steam around 1995.
3738 has been out of traffic for a few years for a boiler overhaul and returned to traffic at Easter 2007. The locomotive was taken out of traffic in July 2013 with firebox problems.