British Rail tube trains

Although the railway network in Great Britain has some of the smallest loading gauges in the world, the vast bulk of it is still capable of operating full sized vehicles. However, British Rail, together with its predecessors and successors have, on occasion, been required to operate passenger trains to an even smaller loading gauge and have, as a consequence, obtained rolling stock identical to that of the "deep tube" lines of London Underground; these are lines built using the tunneling shield method, that were, by necessity, smaller than those lines built using the cut-and-cover method. In 1892, a Parliamentary Committee headed by James Stansfeld recommended that such lines be in tunnels with a minimum diameter of 11ft 6in. Two routes operated by British Rail required the use of such deep-tube rolling stock, the Waterloo & City Line in London, and the Island Line on the Isle of Wight.

The Waterloo & City line was originally built to serve as a link between the terminus of the London and South Western Railway at Waterloo, on the South Bank of the River Thames, and the City of London. Owing to restrictions on the building of railway lines through the centre of London, it was necessary to build this link as an underground railway, with it becoming London's second deep-tube (after the opening of the City & South London Railway). As a consequence of the diameter of the tunnels (12-foot-1 ³⁄₄-inch (3.702 m)), new bespoke trains had to be built which began the service upon its opening in 1898, and continued until 1940.

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