British Railways Class 505 were 1,500 V DC EMUs introduced in 1931 by the Manchester, South Junction and Altrincham Railway (MSJAR). Although assigned to TOPS Class 505 by BR, these units were withdrawn before the TOPS numbering system came into common use for multiple units, and the Class 505 designation is very rarely used.  Following the 1923 Grouping, the MSJAR company was owned jointly by the LMS and LNER. It operated a 13.7 km route between Manchester London Road (now Manchester Piccadilly) and Altrincham in Cheshire. The MSJAR was electrified in the early 1930s on the 1,500 V DC overhead system.

Twenty-two 3-car EMUs and two spare power cars were built for the new electric service, which started on 11 May 1931. These trains were based at Bowdon depot and ran exclusively between Manchester and Altrincham for forty years. In 1939, eight additional trailers were added, both new build and second-hand conversions, inserted into eight of the 3-car sets, and these allowed 7-car trains to be run on many peak hour services. 

In April 1971, all the Class 505 Altrincham Electric units were withdrawn when the line was converted from 1,500 V DC to 25 kV 50 Hz AC. Two centre trailer cars were purchased by the Altrincham Electric Railway Preservation Society and moved to the Yorkshire Dales Railway (now Embsay and Bolton Abbey Steam Railway). In 1983, they were moved to the Midland Railway - Butterley in Derbyshire where they are undergoing restoration. Here they joined centre trailer coach M29663 which had been bought by Derby City Council. This coach was broken up in 2006. No driving coaches have been preserved.