Baffling bi-modes.

Electrification; it's good for the environment and good for train operators and passengers, but it's a very slow, expensive process. Back in the 1960s when the west coast main line was being electrified northwards from Euston, electric locomotives would haul the train as far as Crewe, and later Preston, where a class 50 diesel would take over to pull the train along the yet-to-be electrified sections onward to Glasgow. Once the wires were up all the way to Glasgow in 1974, the class 50s were reallocated to the western region. However, swapping locos is a time consuming business. What if an electric loco could carry its own diesel engine for the bits of track with no electricity supply? The southern region's solution was the class 73 electro-diesel. On the 3rd rail electrified network it worked as an electric locomotive, but where the 3rd rails ended it had a diesel engine to power it. Fast forward to the 21st century and a new breed of electro-diesel locomotive has been introduced: the class 88. It can work off the 25kv overhead wires, or use an on-board diesel generator on non-electrified railways. It could potentially be used with the Mk5 "Nova 3" loco-hauled sets being procured by Transpennine Express, although these will be hauled by conventional class 68 diesel locos initially.
A different solution is offered by the Class 800 IEP train, which is essentially a DEMU fitted with a pantograph. These are intended to replace Intercity 125 high-speed trains. However the Class 800 units are far shorter, coming in either 5 or 9-car sets. Given the inadequacy of 5-car Class 221 Voyager trains on Intercity routes, one wonders why any TOC would bother with a 5-car class 800? But I digress...
A more unconventional electro-diesel comes in the form of the Class 755  train for the Greater Anglia franchise. This is almost the opposite of the class 800, being built primarily as a conventional EMU, but with a separate power car containing four diesel generators. To convert the train from electro-diesel to straight electric, it is simply a matter of removing the power car from the train. As electrification spreads and the need for electro-diesels is reduced, this seems like a good way of keeping a train in service for longer. A class 800 would require considerable time in the workshop to remove all of its diesel engines.