Going from the other posts on this thread, advantages of Multiple Units:
- Distributed power offers quicker acceleration (for diesel anyway, how does a NXEA class 90 hauled train compare to a comparable EMU?)
- Distributed power offers increased tractive effort Counter Point: cuts both ways, a higher weight over the powered axles in a loco probably has a similar effect
- No loss of platform space by having a large power unit at the front/back
- Short trains are possibly lighter and more fuel/energy efficient than LHCS due to heavy, powerful locomotive
and advantages of Loco-Hauled Coaching Stock:
- Long trains are possibly lighter and more fuel/energy efficient than a multiple unit with a motor under every coach
- Much quieter for passengers than a DMU
- Possible vibration from engines/motors that you might get with a multiple unit avoided
- Much easier to change the formation at quiet/busy times of year, or to remove broken cars or replace broken power units without taking the whole train out of service
- Where a train service runs under the wires for part of its journey, swapping the loco on a LHCS train would remove the weight of diesel engines and fuel under the wires, AND the weight of the pantograph and associated gear when on diesel power this is the big one
In the case of XC though it's basically a regional DMU service that also provides some long distance journey opportunities. With the number of stops XC services make these days it wouldn't be practical to use a locomotive. I also doubt the majority of the general public care if the engines are underfloor or not.
How do we know what the majority of the travelling public prefer? Just because we dont hear them complain, doesnt mean they dont care about underfloor engines. They may prefer unpowered coaches, but just have to put up with underfloor engines.
My grandmother was with me on a bus today. She was constantly complaining about the indicator beep every time the bus stopped the music other passengers were playing and a supposed rattle of the exhaust pipe. If she's anything to go by, general passengers DO CARE about a noisy environment, such as underfloor engines for long distance journeys.
Passengers that notice a named locomotive might appreciate the loco, without being rail enthusiasts. However as you say, most would probably not really notice there's a loco (they probably would if it is something like a large logo 37 on one end only (not top-and-tail) with completely contrasting coaching stock (ie. virgin trains liveried stock) but they probably wouldn't care). This majority would probably think, if they used a loco-hauled service "TOC X has nice quiet trains" or "the trains between A and B are nice and quiet". They might not realize it is quiet because they have a loco-hauled train.
For example, if traveling between Southampton and Reading, or anywhere else on the XC network, I think the vast majority of passengers would rather travel on a loco hauled service or an HST than a Voyager, simply because the first two options are better suited for long distance travel than a Voyager. This is because the engine noise of a Voyager gets very intrusive after a while, and Voyagers have too little luggage space (although the same can be said for lots of trains these days, including 158s).
Voyagers have another problem in addition to their underfloor diesel engines and abysmal fuel economy, they are cramped and claustrophobic.
From what I've read on this forum, IEP will have the same engines as Voyagers. They, and any other Intercity DMUs that may be built in the future are going to have worse fuel economy than Intercity 125s, the railway today demands the acceleration of electric trains from a diesel. That's one reason why no more Intercity trains should be built with diesel engines. The woodland trust overturned a government plan that threaten to allow private companies to take over swathes of woodland, which could have resulted in those woods being cut down. That may not be as bad as cutting down rain-forest, but it is along the same lines. Anyway, my point is a environmentally poor government plan was overturned by public rebellion, and I remember Nick Clegg saying "we got this one wrong". We need the government to realize they have got IEP wrong, and stop the environmentally very bad idea of building new diesel Intercity trains.