LNER Thompson L1 Class

The LNER Thompson L1 Class was a series of 100 outside-cylinder 2-6-4 side-tank locomotives designed by Edward Thompson and built by various manufacturers between 1945 and 1950, representing Thompson's attempt to provide the LNER with a modern and capable suburban and secondary passenger tank engine to replace the ageing inherited suburban tanks across the former-GER, GCR, GNR, and NER routes. The L1 was Thompson's most numerically successful new-build design — 100 examples were constructed, making it a significant class despite the relatively short time between Thompson's design work (1945) and nationalisation (1948).

Thompson specified outside cylinders with Walschaerts valve gear for ease of maintenance — consistent with his design philosophy throughout his CME tenure — and a 2-6-4T wheel arrangement that gave six coupled wheels for adhesion and a trailing bogie for stability on the intensive suburban working the class was intended for. The L1 was built to work the Great Eastern suburban services from Liverpool Street, the former-GCR suburban routes, and other LNER suburban and secondary passenger duties.

In service the L1 proved somewhat disappointing: the class suffered from steaming problems and rough riding that limited its effectiveness on the intensive suburban duties it was designed for. Peppercorn improved the design on the final batches, and BR continued building further examples. Despite 100 being built, none was preserved — partly because the class was not regarded as a great success, and partly because the former-GE suburban electrification made them redundant relatively quickly.