E6013, later to become 73110, photographed at Wimbledon on 16 November 1966

The Class 73 is almost unique on British Railways today in that it is capable of being powered by two different sources - traction current picked up from the third rail and also from its on board diesel generator set (albeit at reduced power). As such it is termed an "electro-diesel" (EDL). There was one other electro-diesel type - the class 74 - a later conversion from pure electric locomotive class 71 which was not a success and all have been withdrawn. (The class 92 are also dual powered locomotives - but in this case both supplies are electric - 25kV ac overhead and 750V dc third rail.)

This flexible dual power concept was first considered immediately post World War II as a solution of how to make the most of the Southern's expanding electrified system by permitting working with the same locomotive on non electrified lines in otherwise electrified territory, also on non electrified sections in sidings and when the traction current was switched off during engineering work, etc. Perhaps the electro-diesel concept was copied from the contemporary General Motors FL9 locomotive, but there is a significant difference in that the FL9 is designed and operated with the reverse requirement. That is as a full power diesel-electric locomotive for the majority of the time and only working on electrical pick up on comparatively short stretches of track, mainly the tunnels under the Hudson and East Rivers (where for many years steam/diesel locomotives were prohibited) to gain access to Grand Central Station on Manhattan Island. Also the Southern had its own experience in maintaining electrical drive - albeit only over short gaps in the third rail - with its pure electric "booster" locomotives CC1-3, later class 70.

photograph by Ray Soper

photograph by Ray Soper

photograph by Ray Soper

photograph by Ray Soper