I don't see why supporters of loco haulage are seen as having "rose tinted spectacles" and that units are the only way forward. It's not like anyone is suggesting bringing back 47s and 86s hauling Mk2s, we're talking about a new generation of diesel and electric locos hauling a new generation of coaches.
From the point of view of the passengers, the coaches will be the same as units, in fact my idea would use the "Mk5" coach design for units too. They would be a modular construction allowing various configuration and customisation, as well as easy repairs and changes throughout their lives.
Basically I would build a new generation of coaching stock capable of 140mph. If we want to be really radical, these could be articulated sets (for flexability I would probably avoid full length articulation, but have a "standard" length train formed of say 4 articulated sets, standard being roughly the length of a HST or Mk4 set). There would be two types of Driving Trailer, a 125/140mph capable streamlined on, and a 110mph capable gangwayed one. Like the coaches, these would be on the surface no different to MU driving coaches beyond minor details. As much as possible of the work would be done in the UK, from design through to assembly, only buying "foreign" componants when needed (and even then if possible having final assembly in the UK, if not licenced construction).
To haul these there would be two types of loco, a 125/140mph electric and a 100-125mph diesel. These would likely be based on a foreign design, but again as much work as possible should take place in the UK. Take the "Warships" as an example, built in Britain but licenced from a German design. The electric locos would have sufficient power to give good performance, although on fairly limited stop work Desiro performance would not be needed. The diesels would be much lower powered, but would be geared to balence top speed with acceleration on the particular duties they would normally perform. The coaches they would be hauling would be lighter weight than the EMU some others would have them haul. On any routes which required considerable high speed diesel running, the locos could be used top and tail, HST like. Both the electric and diesel locos would be lightweight, with bogie design to keep track forces to a minimum. Depending on duties, the diesels may be ballasted to provide extra traction.
Operationally, the Electric locos would run push-pull with a driving trailer. On routes where the service continued off the wires the loco would be at the London end. On arrival at the last wired station the Diesel loco would be waiting in the platform and the DT would be driven up to it and coupled. Depending on the requirements, the train may be split with the diesel taking just part forward, or the full set, minus the electric loco. On the return, the diesel would propel with another DT leading. On trains which split, this would be one of the gangwayed DTs. Now, if we're talking new ideas, this is based on the 1960s solutions to the same problem on the London to Weymouth line. Hardly cutting edge, but then as I keep pointing out, Multiple units have been around since the 18th century.
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