The L1 was a Great Northern Railway tank locomotive designed by Henry Ivatt for running commuter trains on the Metropolitan City Lines in London. Although four coupled axles and a driving wheel diameter of only 4 feet 8 inches looked more like a freight locomotive, this design was chosen to achieve the fastest possible acceleration. The boiler and the cylinders were largely the same as the eight-coupled class K goods locomotives. Since the area of operation was in the city and partly in tunnels, a facility for condensing the steam was used. This was recognizable by the pipes from the smokebox to the water tanks.

At first only a prototype was built and it quickly became apparent that the axle load was too high. This can be explained by the fact that the weight of the large boiler was additionally increased by the water reserves and the condensation device. That is why the ten other production engines were created with a slightly smaller boiler, cylinders adapted to it and reduced water supplies. These eleven engines were used in passenger service as planned, but they do not seem to have performed particularly well. While they demonstrated the ability for very rapid acceleration to 30 mph, they did not significantly exceed 40 mph. 

Schematic drawing with dimensions

Locomotive Magazine, August 1903

As the locomotives had great pulling power, 30 more were built in 1905 and 1906 for use on freight trains in West Riding of Yorkshire. In 1907 the eleven others from London were also transferred there. Spread over the years from 1909 to 1926, the locomotives were given a larger boiler that corresponded to the original plans. A superheater was installed on seven and the boiler pressure was reduced from 175 to 170 psi. Locomotive number 131 was experimentally converted to oil firing. From 1923 they became class R1 at the LNER. From 1927 the first engines were retired. During this time there was a proposal to convert the locos to diesel powered compressed air locos. A 400 hp diesel engine would have filled a tank with compressed air, which would then have acted on the existing cylinders. However, these plans were not implemented and in 1934 the last engine was retired.