The Class 455 was originally to be classified as the 510 class, at which point they were planned as a 750 V DC version of the Class 317. However, as the chopper control system at the time was not considered robust enough for the electrically rougher third rail Southern Region, they were fitted with second-hand camshafts and thus classified as the 455 class.
A total of 505 carriages were built by British Rail Engineering Limited’s Holgate Road carriage works and together with 43 existing trailers from Class 508s, formed 137 four-car sets. The 455s allowed the Class 405 and Class 415 to be withdrawn, as well as allowing the Class 508s to be transferred to the Merseyside network for which they were originally intended. They also allowed other stock to be cascaded to the North London and Oxted lines.