The Class 501 units were built by British Railways in its own workshops at Eastleigh on short 57 ft frames supplied by Ashford.

Despite British Railways having recently built modern sliding door trains for electric suburban services in Manchester and Liverpool and on the Great Eastern Main Line (classes 506, 503 and 306 respectively), it was decided that these trains would closely resemble the EPB stock of the Southern Region, which featured individual passenger-operated doors located at each seating bay.

The stock differed from that for the Southern Region in that each vehicle was 57 feet (17.37 m) long instead of 63 feet 6 inches (19.35 m), and the vehicles within the units had screw coupling with two buffers instead of the close-coupled single buffer with chain arrangement used on the Southern multiple units.

Initially these trains were painted in the standard mid-green livery adopted for electric multiple unit stock at nationalisation.

They were never facelifted or refurbished during their career; however, during the 1970s some trains had the intermediate trailers modified from compartment layout to open saloon.

This work was carried out at Croxley Green workshops and, though quite neatly done, the overhead luggage racks which had been attached to the partitions were not replaced, leaving nowhere to put things like coats and umbrellas.

Services between London Euston/Broad Street and Watford and on the Croxley Green branch were subsequently replaced by sliding door Class 313 units.

Furthermore, the London Underground Bakerloo line, which initially shared the system from Queens Park to Watford, was very substantially cut back and ultimately withdrawn north of Harrow & Wealdstone.

The substantial peak services operated by the units from London's Broad Street terminus, to Richmond and Watford, were completely withdrawn during their lifetime.

This was rebuilt to EPB standards and converted for Departmental Use in 1976 as Test Coach “Mars” ADB975032, based at Strawberry Hill, for commissioning refurbished Class 410/411/412 EMUs, but was scrapped on site by MRJ Phillips (contractors) in August 1987.