The SR class LN or Lord Nelson class is a type of 4-cylinder 4-6-0 steam locomotive designed for the Southern Railway by Richard Maunsell in 1926. They were intended for Continental boat trains between London (Victoria) and Dover harbour, but were also later used for express passenger work to the South-West of England. Sixteen of them were constructed, representing the most powerful (although not the most successful) Southern 4-6-0 design. They were all named after famous admirals.

The class continued to operate with British Railways until withdrawn during 1961 and 1962. Only one example of the class – the first engine, Lord Nelson itself – has been saved from scrapping. This has been seen running on mainline tours and preserved railways throughout Britain.

Although the improved ”King Arthur" class 4-6-0 locomotives were capable of the heaviest express passenger work between London and South West England, there was a growth in demand for Continental traffic travelling via Dover and Folkestone. By the mid-1920s the Southern Railway Traffic Department wished to begin operating 500-long-ton (510 t; 560-short-ton) express trains on these routes during peak periods. These would require a more powerful locomotive, able to pull heavier loads at sustained speeds of 55 mph (89 km/h), so as not to impede the congested electrified lines around London. However, any enlargement of the existing 2-cylinder design was not possible due to weight restrictions imposed by the railway's Civil Engineer.

After examining the practice of other British railways, Richard Maunsell, the Chief Mechanical Engineer, secured agreement for a 4-cylinder design, with an improved boiler and Belpaire firebox. The drive would be divided between the front coupled axle for the inside cylinders and the middle coupled axle for the outside cylinders giving better weight distribution and reduced hammer blow. The new design was an inevitable compromise between the need for additional power and to keep the weight down to an acceptable limit.^{[citation needed]}

There were two unusual features of the design: the first of which was the setting of the crank axles at 135°, rather than the standard 90° of other locomotive types. This design necessitated four sets of valve gear, and gave rise to eight beats per revolution, rather than the usual four, designed to give a more even draw on the fire and less chance of wheelslip when starting. The second difference was that fire grate was in two sections, the rear portion was horizontal and the front sloped away sharply.

The prototype E850 named Lord Nelson was ordered from Eastleigh railway works in June 1925 but production proceeded slowly, at Maunsell's insistence, to ensure that the weight was kept to a minimum at every stage, so the locomotive did not appear until August 1926. It was tested on a variety of duties over the next year, with sufficiently encouraging results for an initial order for ten more locomotives for delivery between May 1928 and April 1929 to be placed. These were originally scheduled to be allocated to Battersea depot and fitted with 4,000 gallon 6-wheeled tenders suitable for the Continental ports. However, during construction, it was decided to equip half of the class with 5,000 gallon 8-wheeled tenders necessary for the longer West of England routes and to allocate them to Nine Elms depot. A further batch of ten locomotives was ordered in 1928, before the previous batch had been delivered, but when it became apparent that the Stock Market crash of 1929 would be likely to reduce the demand for Continental travel, this second order was reduced to five.

The locomotives were all named after famous Royal Navy admirals, with the first of the class being named Lord Nelson. As a result, the rest of the locomotives belonged to the Lord Nelson (LN) class.

The performance of the new locomotives was mixed, depending upon the experience of the crew and the circumstances under which they were operating. At times it was no better than their smaller predecessors. Maunsell therefore undertook a number of experiments to try to improve the performance of the new locomotives. No. E859 was fitted with smaller 6 ft 3 in (1.905 m) driving wheels to see if this would improve performance over the heavily graded London-Dover line, but the difference was marginal. No. E860 was fitted with a longer, heavier boiler but once again with little improvement. The whole class however benefitted from the fitting of smoke deflectors during the late 1920s.^{[citation needed]}