Traditional TDM (although this is pointless now as it's only 90s, 47s and 57s that use it, but it's a good use for the 90s, plenty of life left in them!)
How could you forget the 91s, surely they have TDM! Sadly I think you are wrong about 47s and 57s having TDM (47/7s had it once, and might still do so, but I've been told on here that it was an incompatible form of TDM that was only suitable for use with DBSOs which were then converted to work with 87s or 86s and 90s). It means we can't use LHCS on diesel routes without modifying DVTs to other control systems, rather than just being able to stick the current DVTs and DBSOs on a train as they are. Now if we converted another batch of 47s to 57s, with TDM and a greater focus on making something reliable this time, we might be able to get things moving.
The silly thing is, diesel locos are always more eco friendly. But you have a handful that try and claim voyagers etc use less fuel. Even a 47 and 7/8 coaches uses less fuel than an 8 car voyager. But its to be expected. Physics tells you that 1 loco is going to be more economical than 7 or 8 powered coaches.
Its only when you get to the electrics that there is very little difference. You still get those claiming EMUs are better, but thats only when they compare a fantasy EMU (ive seen someone try and explain that a pendy could be ultra amazing if you improved it for heavens sake) to an 80s electric loco. Compare like for like, and there will be next to no difference. Unpowered coaches will still be quieter though.
Take a 1990 locomotive (class 91), nine mark 4 coaches and several tons of practically wasted space (the DVT) and it matches the 'fuel' economy (per seat km) of a 9-car Pendolino. Turn the Pendo's regenerative braking off to level the playing field and the IC225 will probablly take the lead (that would be a silly thing to do though, put seats in the DVT though and the Pendo's fuel ecconomy per-seat should be beaten soundly). However, you can't really compare like-with-like, since nobody's ever built an EMU which is similar enough to an existing LHCS rake, or vice versa.
With a bi-mode MU though all you have to do is remove the engines once electrification of the route is complete. You wouldn't need to build a new electric loco.
So you end up with sky high development costs to incorporate this new engine and then a pile of useless engines in 10 - 15 years, good play...
Maybe longer than 10-15 years but due to climate change diesel engines are not likely to be an option in the long term so best to plan for the possibility of running without them.
So why not save the cost of building the diesel engines, and the climate change impact of running them, and have a diesel-free IEP EMU or electric LHCS rake?
But thinking sensibly, yes, an IC225 type train. DVT at the end with no passengers in it. Gives space for luggage and bikes etc which we wouldnt find space for if it was full of passengers.
An IC225 like train would be great, especially if we had the afformentioned reliable TDM-fitted 57s which could take over from the electric loco away from the wires. However, I disagree that the DVT should still be empty, use it for passengers so we can have more legroom throughout the train and have at least as many seats in total. There would probablly still be the passenger-free crumple zone in the DVT which I'm sure could have a few bikes or suitcases parked in it.
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Anyway, far more useful right now than trying to specify the next generation of LHCS stock I think would be to find a way to make the DfT drop all diesel engines from IEP. Then, to keep Hitachi and the proposed workers at the north-east's assembly plant happy order a climate-friendly diesel-free Hitachi IEP EMU fleet for some (maybe all) East Coast routes, and Paddington-Oxford/Bristol/Cardiff (Swansea too if you can't get the LHCS) and Cheltenham.