Mersey Railway Electric Units

Mersey Railway electric multiple units were electric multiple units introduced on the underground Mersey Railway in 1903. In the early 1900s the railway was bankrupt as it used steam locomotives that left a dirty atmosphere in the tunnel and passengers preferred the ferries. However, the railway was rescued by Westinghouse Electric, who electrified the railway and provided the first electric multiple units. The cars were supplemented in 1908, 1923 and 1925 and finally in 1936 to allow the progression from 4-car, through 5-car, and finally to 6-car trains. In 1938, when the Wirral Railway was electrified, the units were modified to allow through running between the two systems. In 1956–57 the cars were replaced by units similar to those used on the Wirral Railway.

Under LMS they were known as the AM3 class and under BR's TOPS system, they became the BR Class 503.