In the early twentieth century the North British Railway possessed an ageing locomotive fleet which had not kept pace with modern demands.

[1] The NBR Board met on 2 November 1905 and approved the construction of 14 heavy express passenger locomotives, with design work to commence immediately.

Many railways were, in this era, constructing powerful express passenger locomotives of the 4-6-0 type, which benefit from 6 coupled driving wheels.

The North British Railway's principal passenger engines had hitherto been of the 4-4-0 type, meaning that 4-coupled drive was more familiar.

Additionally, the NBR had some very tight curves, for which the use of the "Atlantic" type engine with its 4-4-2 wheel arrangement was simply more practical and efficient.

Having established themselves as the most powerful passenger locomotives on the North British network, but being insufficient in number to operate all of the crack express services, a further 6 engines were ordered and constructed during 1911.

[citation needed] The North British Railway was overly ambitious in its advertising of the new locomotives, and despite rapid construction, the engines were not ready in time for the new timetables which they had been designed to serve.

Early concerns about their weight (as expressed by James Bell) and their centre of gravity (leading William Jackson, NBR General Manager, to order an expensive "swing test" of an engine in January 1907[clarification needed]) proved unfounded.

So that the national collection was not denied a Class H engine, Midlothian (whose component parts were mostly still in existence, and frame still fully intact) was painstakingly rebuilt, and returned to service for transfer to the LNER Railway Museum at York (which later formed part of the basis of the National Railway Museum).

The newly rebuilt Midlothian was withdrawn from the museum stock, and scrapped for a second time, to provide aircraft-building materials.