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.

Work on the dual power concept continued throughout the 1950s, during which time the straight electric class 71 was also designed and introduced. The class 71 attempted to solve the absence of third rail issue with additional catenary pick up but the overhead lines were only installed in a limited number of siding locations.

Finally an order for six electro-diesel locomotives was made in July 1959. These were initially designated class JA, subsequently under TOPS initially Class 72 but Class 73/0 was the classification that was implemented. They were numbered E6001-6, subsequently N°73001-6. These six were built at the SR's Eastleigh works and are capable of 1600hp (i.e. similar to the Southern's native diesel electric class 33) on straight electrical pick up and 600hp from its own diesel-electric generator set. In tune with the Southern's policy of standardisation and interchangeability of parts the diesel-electric generator set was from the same series as used in the DEMU units and the traction bogies (albeit with different pick up arrangements) as on contemporary EMU and DEMU stock. The class was designed to work in multiple with all other Southern MU stock - with other JAs, EMUs, DEMUs and subsequently JBs, 33/1s and 74s. A strong point of the design is that the locomotives can change their power at speed since the diesel can be started and stopped and the pick-ups raised and lowered on the move.

The first JA emerged from Eastleigh on the 1st February 1962 and all six were in service by the end of the year. So successful was the design that an order for a further 30, subsequently 43, was made. These were built by the English Electric Company at their Vulcan foundry Works in Newton-le-Willows. The first of this series, designated JB (later Class 73/1), E6007 left the works on 13 October 1965 and all were in service by January 1967. The JBs are distinguishable from the JAs since they have one less high level MU jumper cable and one less body side window. The JBs also have different traction motors allowing a top speed of 90mph - 10mph higher than the JAs.

The Class 73s staple work initially was on freight, parcels and departmental services with only limited passenger workings such as boat trains and overnight services. However the class provided an interim service on the Bournemouth line pending arrival of the 4-Reps and introduction of the full electrified service. Indeed the Class 73s reprised this role - somewhat in a reverse scenario - in the late 1980s when they substituted for withdrawn Rep power cars in makeshift units so the Rep motors electrical equipment could be re-cycled into the new class 442 units.

As already mentioned such was the success of the EDL concept that in 1967 10 surplus class 71 straight electric locomotives were converted into "big" EDLs capable of a higher power output in both modes. These were primarily intended for the heavy Southampton Boat Train traffic and also through services to Weymouth Quay. However they proved to be consistently unreliable and were withdrawn after only 10 years, leaving the Class 73s as they started - the sole EDL class.

The first regular prime passenger workings for the Class 73/1s came in 1984 with the commencement of the premium Gatwick Express service. The class was selected to power in a push pull mode mark 2 trailer formations (class 488) with a powered Gatwick Luggage Van (GLV/class 489) on the other end. Initially locomotives were not dedicated to the service and any Class 73/1 could be called upon.

It was whilst working this intensive service that the only major problem with the class emerged. A problem associated with electrical locomotives picking up from the third rail is that the length between their pickups is considerably shorter than that on an EMU. Thus the locomotive can lose traction power - become "gapped" - when the length of a gap in the conductor rail exceeds the length between the locomotive's pick ups. This is commonly experienced through complex pointwork. The Southern's pioneer electrical locomotives CC1/2/20003, and the class 71 overcame this by the traction power driving a motor generator set in which a large flywheel was inserted, thus earning the pioneer locomotives the nickname "boosters". However this solution was not adopted on the Class 73s (and removed when the 71s were converted to 74s). The Gatwick Express route encounters a number of significant gaps particularly in the Battersea Park, Purley Oaks, Stoats Nest and Coulsdon areas and it has to be remembered that in the Gatwick Express unit configuration there is no connection of traction power - other than via the third rail - between the EDL and the GLV. Two minor fires in Class 73s in the Battersea Park vicinity culminated on the 5th August 1984 in a serious fire in the celebrity "Royal" EDL N°73142 Broadlands. This resulted in the temporary withdrawal of the Gatwick Express formations and substitutions by EMUs until the cause could be traced and rectifications made. The cause of these fires was arcing within the locomotive due to adjacent conductor rails at gaps having differing voltages. Class 73s were fitted with flashguards to their bogies and modifications to how the control gear operates when traction power is lost and then restored. N°73142 was rebuilt and returned to service.

Another prime and exclusive passenger working by the class is powering the British VSOE trains around the Southern Region. This resulted in N°73101 being painted into Pullman livery. In February 1988 (ahead of assignment of assets to the Business Sectors under BR's 1991 Organisation for Quality) a dedicated pool of Class 73s for Gatwick Express services was created with the locomotives being re-designated class Class 73/2. Initially of 12 this pool grew to 14 and in the early days the pool provided traction to VSOE services as well. In fact it took some time after the pool was created for Class 73/2s to be confined to their own services and for Class 73/1s not to appear on Gatwick Express workings. It was originally mooted that the locomotives in this sub class would have their diesel engines removed as the sector's requirement was for straight electric traction but in the end common sense prevailed and the diesels were retained to get the service out of trouble on many an occasion. Class 73/2s have minor detail differences to Class 73/1s in that their vacuum brakes have been isolated and connecting hoses removed, as have low level MU connections from their buffer beams.

The final two major changes to Class 73 use firstly came with the allocation of N°73118 and N°73130 to European Passenger Services/Eurostar and these two have their front end modified to carry pivoting Scharfenburg couplers to enable them to both shunt and rescue Eurostar sets. Then secondly with the assignment of five of the Class 73/0s to the Merseyrail system largely for infrastructure work. Two of these N°73001 and N°73006 have been converted for Sandite duties and have been renumbered into the Class 73/9 series as N°73901 and N°73906 respectively. A current "star" of the fleet is N°73109 which is the South West Trains "Thunderbird" which is normally based at Woking to travel throughout the SWT network to rescue failed trains.

A number of Class 73s have been named and some later de-named. As for liveries - it is inevitable in such a long lived class that a wide range of liveries have been carried reflecting the changing colour schemes over the years. However the Class 73s have also had some liveries of their own. For modelling and historical research purposes you are strongly advised to seek documentary and pictorial evidence of your chosen locomotive(s) at chosen dates. The JAs were out-shopped in green with small yellow warning panels although soon the warning panels were removed for a time and a light coloured band applied at solebar level, then the warning panels were later reapplied. As to the shade of green and the light coloured band be warned published accounts vary. The green has been described as Brunswick Green or a lighter shade being either malachite or BR(S) coaching stock green. The light coloured band has been described as grey or light green. Photographic evidence indicates the JAs wore light green with a grey band. The shade of green is likely to be that also carried by the class 71s and one thing is for sure is that the JAs carried the coach roundel rather than the BR emblem. The JBs were initially painted in the pleasant "electric" blue - as also worn by the contemporary AC electrics - with a light grey band at solebar level and small yellow warning panels. Published accounts are inconclusive whether all JBs were out-shopped in electric blue or just the early ones. Thereafter the warning panels became full height and the whole fleet had succumbed to overall blue BR corporate livery by 1970. Thereafter, but with some variations on individual locomotives, the class has worn a procession of "large logo" blue, original Intercity, revised Intercity "Swallow" livery (Gatwick Express Class 73s only), Gatwick Express Livery, departmental overall grey, departmental "Dutch" livery, Pullman chocolate and cream, Eurostar two tone grey with blue roof, Mainline Freight blue, EWS red and gold, Network SouthEast overall blue, full Network SouthEast colours, Merseyrail yellow and South West Trains livery.

Most of the class have been withdrawn although EWS had a number stored. N°73003 has been preserved by the Electro-Diesel Group and runs on the Lavender Line in Sussex. Merseyrail sold N°73004 to the EDL group to be scrapped for parts to maintain N°73003. N°73111 was scrapped some time ago whilst still in overall blue livery. Unfortunately with the replacement of Gatwick Express push pull working by class 460 EMUs and the preference for diesel-electric traction for freight working even on electrified lines means the days of this highly useful flexible (if unsung) class were numbered. However N°73202 has been retained by Gatwick Express and it has had its draw gear converted to be able to rescue disabled class 460 units.