In their original form the locomotives did not look much like this, being side tanks with no back to the cab and intended to be used in pairs operated by a single crew. They were built by Sharp Stewart for the Cornwall Minerals Railway. The designer is a little obscure. Its apparently credited to an F. Trevithick. Francis Trevithick, son of the great pioneer, had formerly been Locomotive superintendent of the Northern division of the LNWR and was resident in Cornwall at the time working as Factor for the Tehidy Estate, which had considerable mineral connections. One of his subordinates had been Alexander Allan, inventor of the eponymous valve gear, with which these locomotives were fitted. There were other F. Trevithicks, but he seems to be considered most likely.
The GWR took over running the line in 1877, but only acquired nine of the line’s eighteen identical locomotives as the other nine were pledged as security against various debts and were sold separately.
The GWR numbered their locos 1392-1400. In 1883/4 they were all converted to saddle tanks and given a rear frame extension to provide a conventional cab and bunker. They received a variety of cabs, tanks and bunkers over the years and were twice reboilered, the second time with 1361 class boilers, but were otherwise little altered. One was sold in 1883 and 1392 scrapped after a collision in 1906, but otherwise they survived into the 1930s. After 1392 was scrapped the class became known as the 1393 class! At the 1912 renumbering 1400 was renumbered 1398, being the number of the loco sold in 1883.
In GWR history they were significant as being the basis of the design of the 1361 and 1366 classes. Harry Holcroft tells the story of "a roll of musty old drawings" being deposited at his drawing board, which were those of the 1392s, which he was instructed to use to design a complete new class.
This sketch is partly based on a 19thC weight diagram which is minimal in the extreme, and partly on 20thC photographs. I think its hopefully reasonably representative of an 20thC configuration for the class, although I've had to rely a little more on the known similarity to the 1361 class as is perhaps advisable. Later weight diagrams exist and it would be interesting to see those to try and tie things down a little more.
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