The British Rail Class 502 was a type of electric multiple-unit passenger train, originally built by the London, Midland and Scottish Railway at its Derby Works. Introduced in 1940 and withdrawn by 1980, they spent the whole of their working lives on the electrified railway lines north of Liverpool. The trains were designed to replace older electric trains built by the Lancashire and Yorkshire Railway on the lines from Liverpool Exchange railway station to Southport and Ormskirk. These lines were electrified with a direct current (DC) third rail. The Class 502s entered service between 1940 and 1943. They were DC-only and operated as both three-car and two-car sets, which could be coupled together to form five-car or six-car sets for use on the busier services. Design A very modern design for the time, they were equipped with air-operated sliding doors. They were similar to (but somewhat larger than) the Class 503s operating in Wirral, being both longer and wider, allowing 3+2 seating on the Southport 502s compared to 2+2 seating on the Wirral 503s. In addition the 502s had distinctive large resistance cooling grids mounted on top of the cab roof of the motor units. Virtually all electric suburban passenger trains in Great Britain now follow the basic layout pioneered by the Class 502s and 503s, with two sets of double-leaf air operated sliding doors on each side of each carriage. Build Unlike the Wirral Class 503s, which were built by contractors familiar with this type of layout on London Underground trains, the 502s which followed shortly afterwards were built by the LMS's own workshops in Derby. The 502s had conventional railway buffers at the end of each set, as they did a considerable amount of interworking with steam and later diesel services, whereas the 503s in Wirral had automatic Buckeye couplers and no buffers, as they did very little inter-running with services from outside. The Class 502s were considerably more powerful than their Wirral counterparts, having four traction motors. Interiors The interiors of the Class 502s contained varnished wood, tungsten lighting and deep-sprung seating. Apart from the construction differences with the Class 503s, both types of train were given heavy overhauls at Horwich works, and the common red/black/grey seat moquette, and grey paint used on the interiors, plus other common details, gave them a considerable air of similarity. Route changes Liverpool Exchange station closed in 1977, being replaced by