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). They were designed for, and operated on, the Wirral & Mersey lines from Liverpool to West Kirby, New Brighton, and Rock Ferry.