text: The limitations of a 1,600 hp electric locomotive meant that it was only capable of hauling maximum ten-coach trains to the proposed post-steam era Southern Region express timings, as were the similarly powered Type 3 BRCW Class 33 diesel locos with 1,550 hp. Southern express electric multiple units had, by comparison, at least 1,000 hp per four-car unit. To assume that the Class 73 was suitable to haul passenger or freight trains away from the electrified network with just a 600 hp diesel engine would be unrealistic. In fact the use of the slightly more powerful Class 74 electro-diesels, with 650 hp on diesel, to haul parcels trains up Poole Bank and work freight trains around the West London Line to Acton, Brent and Willesden, resulted in repeated failures; this was, therefore, a major contributory factor to that class's early demise.
  There were eight third rail collector shoes on the Class 73 – four per bogie with two each side. These shoes were of the retractable type – a necessity for both safety and operational reasons, as described above. They could cope with all the then (1965) voltages of 660V and 750V as well as the 800V of the Bournemouth electrification beyond Pirbright Junction south of Woking (from autumn 1966). 660V was phased out in line with pre-BR type electric multiple unit (EMU) withdrawals, the last EMUs incapable of working in passenger service on anything greater than 660V being the 4SUB Bulleid design. Thus all BR post-1951 EMU designs could cope with 750V or 800V.