In 1945 Edward Thompson produced the first 2-6-4T Class L1 locomotive at the LNER’s Darlington Works. Designed for mixed traffic, short distance passenger and freight services, these locomotives had a 5'2" diameter driving wheel, two outside 20" x 26" cylinders, a boiler pressure of 225psi and a weight of nearly 91 tons. Prototype No. 9300 underwent extensive tests before more of the class were put into production, with the remaining 99 being built between 1947 and 1950. Not released into traffic until after the formation of British Railways in 1948, the first three of the Class actually entered traffic in LNER livery, bearing the LNER numbers 9001, 9002 and 9003.
This was changed to British Railways livery and numbering within a few months, the class being re-numbered twice by British Railways.Unfortunately the L1 Class did not live up to expectations; whilst the locomotive had adequate power for hauling heavy freight trains at slower speeds, they were mainly assigned to outer London suburban passenger services where faster speeds and frequent stops wore the engines out very quickly. High maintenance costs led to various modifications on some of the engines in an effort to improve performance and reliability, but the modifications did little, if anything to help. Very careful handling was required by the crews who soon nicknamed the L1 as 'Cement Mixers', due to the loud clanking noise they made.