In 1939, the LMS introduced a new type of EMUs for service on their third-rail DC lines from Liverpool to Southport and Ormskirk. They were similar to the AM3 (later class 503), but much more powerful. Four traction motors with 235 hp each were located in one car. Each unit was composed of two or three cars.
By 1941, 34 three-car and 25 two-car units had been completed. Since the two-car units only had one cab, they could only be used together with a three-car unit. For all of their lives, they were used in the Liverpool area. From 1978 they were replaced by the class 507 and withdrawn by 1980.