Hail All!
How will your dual engined 43/5 (and why /5?) be fuelled? At those kind of speeds, I'm thinking a nuclear train or a Hydrogen fuel cell train...
Well, what I had in mind was actually a re-take on the HST design, with each converted power car having two Valenta engine and alternator sets. One of these would be used to power the traction motors local to that power car, and the other one would be used to supply power to the Class 91 at the other end via a busbar arrangement through the train. 8)
In the interim it'd be Diesel powered just like a normal 43, with a possible switchover to used cooking oil at a future point pending experiments on using such "fuel" in Diesel engines.

(And I chose /5 as a sub-class as I thought there must've been a revision or two since 43/0s came out and I was too tired to Google it. Indeed - Assuming that traditional Valenta'd HSTs would've been a 43/0 - Wouldn't that make MTU'd HSTs a 43/1 class? :?)
I'm not so sure about the potential for high-speed Hydrogen powered vehicles (I don't know much about Hydrogen-powered traction) but Nuclear power would certainly allow for some impressive speeds to be reached in relation to the reactor's size. The several headaches though - Going on what I know of the Magnox design of reactors - Would be:
- Trying to design and build a suitable nuclear core that would fit inside a C1 loading guage (Virtually impossible),
- Designing a suitable core would be light enough to be transportable by rail (Definitely impossible - Normal nuclear reactors use a LOT of lead for shielding!), and
- Most importantly: Would be strong enough to survive a serious incident at highly overrated speeds (Such as 1,500mph for a train designed to achieve 300mph) without any form of leakage from the core whatsoever.
Although the idea of Nuclear-powered traction is a nice one from the viewpoint of available power and resulting speeds, the major safety issues and inevitable public reaction to such a system being used pretty much consigns the idea of onboard Nuclear power to the bin before it's even got off the drawing board.

hock:
(Although nothing necessarily unsafe in having a static Nuclear station supplying power to the OHLE, though...I'd imagine that it already happens!

)
Aberdeen - London is over 500 miles - no way you could do it in 4 hours.
Eh? I always had London - Aberdeen down as about 300 miles!

hock:
Using the example that I described above, that would mean a non-stop journey time of five hours at an average speed of 60mph, or three hours at an average 100mph. 8)
Of course the reason that I specified a
motorcycle (And not my normal preferred Vanhool AGG-300 or Plaxton Paramount) is because traffic queues, delays and roadworks could be factored out of an "ideal time" calculation.
Maybe Death has a "slightly modified" motorcycle in the same vein as the "slightly modified APT" he likes to talk about, therefore capable of 500+mph?
I certainly
wouldn't mind having a motorcycle capable of 500+mph, but - Having seen an unsuccessful speed record attempt using a rocket or gas-turbine powered motorcycle - I don't think that it's possible for a motorcycle-style vehicle to exceed 300mph or so. Above those speeds, the vehicles natural tendancy to "hunt" becomes so pronounced that it'd be impossible to keep the vehicle upright without a very well engineered gyroscopic stabiliser...

hock:
Farewell...
>> Death <<