British Rail Class 503 trains were 65 mph (105 km/h) electric multiple units. They were introduced in two batches. The first were in 1938, by the London, Midland and Scottish Railway (LMS) with a further batch (built to a similar design) in 1956 by the then nationalised British Railways (BR). When introduced by the LMS, they were officially known as Class AM3.

They were designed for, and operated on, the Wirral & Mersey lines from Liverpool to West Kirby, New Brighton, and Rock Ferry. There were few places on their network of closely spaced stations to attain their maximum speed, except for the open section between Moreton and Meols.

All but one set were withdrawn and scrapped by 1985. The final set was used on special Merseyrail services until 1988, and this preserved set was kept at the Electric Railway Museum near Coventry until moved to the Locomotive Storage Ltd warehouse at Margate. Though never carried on the unit, the set was numbered under the BR TOPS code as 503 019.