The British Rail Class 501 electric multiple units were built in 1955/56 for use on the former LNWR/LMS suburban electric network of the London Midland Region. A total of 57 three-car units were built. Services The services worked by the Class 501 units consisted of a small self-contained group of lines, which were electrified at 630 V DC on the 3rd and 4th rail principle, linking London Euston to Watford, Broad Street with both Richmond and Watford plus the Croxley Green branch — which was mostly served by shuttle trains from Watford. Some of these services were partially jointly operated with London Underground's Bakerloo and District Lines. In 1970 the trains were converted to 3rd rail only operation, although the 4th rail was retained on sections of line which are shared with London Underground trains. The original electric scheme on these lines was at 630 V DC, which was actually supplied at +420 V in the outer electric rail and -210 V in the centre one, a total potential difference of 630 V. This was the same system as used at the time (and still employed today) by London Underground, and allowed Underground trains to run over the lines, from Queens Park to Watford (Bakerloo line) and Gunnersbury to Richmond (District line). In 1970 it was decided to convert to the normal arrangement of the full positive current of +630V in the outer rail, and to use the normal running rails for the return current at 0 potential. This required adaptation of the Class 501 trains to return current through the wheels instead of the centre rail. On the sections shared with the Underground trains the centre rail was retained, but was now also at zero potential and just bonded electrically to the running rails. No modifications were required on the Underground trains for this change, but electrical switching units are at the lineside where Underground trains pass from one system to the other. Description 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 , and 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