The London and North Eastern Railway (LNER) Thompson Class L1 was a class of 2-6-4T steam locomotives designed by Edward Thompson. The prototype no. 9000 was built in 1945, but the remaining 99 were built under British Railways jurisdiction between 1948–1950. The prototype was well received, however the production batch were not, and all were withdrawn and scrapped between 1960 and 1962.

The class was designed to address the lack of modern tank engines on the Central and Eastern sections of the LNER, replacing the six Metropolitan Railway K Class and to serve alongside the eighty Gresley V1/V3 tank engines. Under Edward Thompson, who sought to standardise on the large number of locomotives of the LNER, the L1 was designed to be able to carry out both passenger and goods work.

Similar to other Thompson designs, the design reused many standard components. The cylinders were the same as the B1, with a boiler based on the V3 design uprated to 225 psi. The engine had a large water capacity at 2360 gallons and a coal capacity 4.5 tonnes.^[2] The 5 ft 2 in (1.575 m) driving wheels and boiler pressure gave a tractive effort of 32,080 lb. To operate on the Eastern section, Westinghouse brakes and vacuum ejectors were fitted.^[2]

Aiding crew comforts were electric cab lighting, electric head and tail lights, folding discs and good visibility when running bunker first. The running plate features a cutout gave good access to the motion, easing maintenance.^[2]

Both the production batch from Darlington and Robert Stephenson and Co. incorporated minor changes from No. 9000. This ranged from style of the cab doors, cylinder linings, boiler pressure to fabricated axle boxes. It is noted that the axle boxes and water tanks were made using welding.^[3] These emerged under British Railway auspices, after both Thompson and Peppercorn has retired.

In an attempt to solve the overheating axle box issue, two experiments were tried. In May 1951, five locomotives had liners fitted to their cylinders to reduce the cylinder bore from 20 to 18+3⁄4 in (508 to 476 mm). In March 1953, five locomotives had their boiler pressure reduced from 225 to 200 lbf/in² (1.55 to 1.38 MPa). Neither experiment was a success. Simon A.C. Martin asserts that the reason for the overheating axle boxes was due to leaking water tanks, and that the issue was resolved by making sure they were watertight.^[4]

The first of the class, No. 9000, was the only member of the class to be built by the LNER, and emerged in May 1945 from Doncaster painted in LNER Apple Green.^[5] The engine was well received with praise from enthusiasts and railwaymen at the time, and underwent the most extensive set of trials with an LNER locomotive, bar the LNER Class W1.^[6] It hauled every type of train possible, mostly with the dynamometer car recording results.^[6]

The small 5 ft 2 in (1.575 m) driving wheels limited their maximum speed, but were ideal for accelerating from rest or on gradients, such as the London suburban area.^[5] Coal and water consumption were lower then that of other locomotives, but it was also capable of pulling greater loads.

Both O.S. Nock and Peter Grafton remarked that the prototype achieved a punctual run on a 366 long tons (372 t; 410 short tons) express passenger train between Liverpool Street and Ipswich at a economical water consumption of under 30 imp gal (140 L; 36 US gal) per mile; in addition to the 386 long tons (392 t; 432 short tons) return train consuming less than 0.1 lb per train ton mile. The two both remarks that such a feat was not intentional, and could only be considered exceptional.^{[7] [2]} The prototype was also trialed on coal trains, although its insufficient braking capacity and adhesion mitigated against its use on said services.^[2]

She managed only just over 10,000 miles in the latter half of 1945, but improved substantially to around 28,000 miles in 1946, matching Gresley V1s and V3s.^[8] Both it and Gresley V1s had similar availability numbers at 79%, higher then the higher pressure V3 design with 68% availability.

The rest of the class were ordered under Arthur Peppercorn, who ordered minor changes to the production L1s, and would emerge under British Railways.^[3]

During widespread service, they never achieved the same appreciation as the prototype, with complaints such as "shaking themselves to bits", "concrete mixers" and "too small driving wheels".^[3] The production batch was also said to be more temperamental then their prototype. Due to a lack of improved draughting and fitting of a self-cleaning smokebox, they had to carefully handled by crews.^[2] L1s were used on empty stock workings, and reportedly struggle to move empty sleeping car trains out of King's Cross.^[2]

It is also said that their axle boxes suffered from overheating. Modifications were made to solve the issue, such as reducing the diameter of the cylinders and reducing the boiler pressure. However, the modifications seemed to not have been successful, and the class suffered from the issue until their withdrawal. Although Simon A.C. Martin claims that the issue was caused by leaking water tanks introducing detritus and removing lubrication, and was largely solved with axle box shims and making sure the tanks were watertight.^[4]

• On 19 November 1958, a freight train overran signals and was in a rear-end collision with another at Hitchin, Hertfordshire. A third freight train ran into the wreckage and was derailed. Locomotive No. 67785 was pushed over by the wagons from the third train.^[9]

Withdrawals were between 1960 and 1962. None survived to preservation.

Hornby produces the L1 class in 00 gauge with a number of different liveries, both green with LNER or BR running numbers,^[13][14] and black with BR running numbers.^[15][16]