The efficiency gain in splitting trains is mostly related to reducing the number of drivers needed. It also reduces the demand on track and platforms that may be congested. Both these things could be solved through better automation. Have the train driven by a computer and automate signalling to allow trains to run closer together.
Some inner-city rail in Europe is like you describe but not anything high speed (>~100mph) AFAIK.
However everything else you've claimed is wrong. The designs you're talking about DO have motors under many (or in some cases all) passenger coaches driving the wheels, rather than a dedicated "locomotive" so they support your parent's point, not yours.
The original Eurostar (class 373) trains are confusing because they're built as multiple units but the lead and trailing cars basically just provide traction, so the result looks a lot like a "conventional" locomotive plus carriages. But modern Eurostar (class 374) looks exactly like any other multiple unit and has motors distributed across the whole train.
Britain does operate a bunch of locomotive pulled high speed (125mph) services, but it also owns a bunch of 125mph multiple units. Even the Desiro 444 and 450 trains could do 125mph if not for the fact they're configured exclusively for third rail power limited to 100mph.
The reason none of those trains exceeds 125mph is purely down to legal/ safety considerations. Britain uses coloured light signals with route control, as speeds increase drivers struggle to correctly interpret and act on a light signal by the side of the track. HS-1 (Eurostar) and many European high speed services have in-cab signals, so the driver doesn't need to try to observe fixed coloured light signals at the side of the track, the indications are inside the cab with them.
As well as the reasons given earlier about improved performance from distributing the traction power across the train, distributing the weight also improves crash performance. Heavy locomotives will tend to crush a lightweight passenger car, if we make all the cars of more similar weight this problem goes away.
Eh? I didn't say anything at all about where the motors were.
You're the second person in this thread to imagine me saying something wrong and call me out on it.
> Some inner-city rail in Europe is like you describe but not anything high speed (>~100mph) AFAIK.
That's referring to a post which claimed:
> most high speed electric trains have motors in every car because electric motors scale down just fine
The examples you gave were ICE and the Eurostar, which are groups rather than specific classes of high speed train, but in both cases the modern classes have motors in every car, they specifically _disprove_ your claim.
There are also electro-diesels which can shut off their diesel engine and take electricity directly from an overhead line or third rail by raising a pantograph or lowering a boot. This used to happen at Faringdon in London (maybe still does).
Diesel electric transmission does happen on smaller multiple units sometimes, but really shines on larger and faster trains, or on electro-diesels (where you need the electric final drive anyway).
Some trains are hybrid - will use electricity while under wires, and switch to diesel when there's no wires, the new class 800 in the UK as an example.
Some 319s are being converted to partial diesel and becoming class 769s, but not on this route.
Before 2009 and HS1 to St Pancras, Eurostar also switched from overhead to third rail, on the fly I think, somewhere near Ashford.
Certainly in the UK: the most common locomotives such as class 66, 60, 58, 67, 47 - they're all diesel electric.
In the US, same story: for passengers the GE Genesis is pretty big, things like SD90MAC and AC6000CW are big for freight.
https://www.youtube.com/watch?v=2OGRwD-00b8
https://patents.google.com/patent/US20030230213
Not sure if this is what jarfil meant by "steering".
It would be kind of tedious to go into great detail. Feels like nerd sniping.
But in order to achieve higher granularity and better schedules, cars would have to become more autonomous, which means duplicate steering systems and lower efficiencies... unless there were no efficiency benefits in having a dedicated locomotive, in which case it would make no sense to join cars together in the first place.