However much you may wish otherwise the franchises are commercial businesses that aim to make a profit. They will stick rigidly to the franchise specification unless they see a profit in going beyond it. In this case the extra costs of running HSTs instead of Voyagers would have to be outweighed by the extra revenue from getting more passengers. This is particularly complicated if extra HST journeys mean they can no longer be serviced at a single depot or if there is a major mileage element in the leasing/maintenance costs.
Like all commercial businesses bidding for work, if they include some "fat" in their bids to provide "nice to haves" beyond what the specification says, then they will risk losing out on price to the bidder that didn't do that.
makes a lot more sense running a large uniform fleet of small multiple units times x then as a fleet,that way they can mix and match their runnings/loads to times of day/seasonal variations.
I agree from a train operators point of view it is not a good business plan running 2/3 empty HST sets for 2/3 of the day....but you want to maximise capacity at the same time.
would be much better running a 3*3 car multiple unit at peak, one of which can be redeployed as a rescue/holiday surplus stock as and when location and footfall demand.the TOC's must have figures somewhere to know what routes are busy at what time of year.
the prerequisites to the stock would be able to cope with different duty cycles...if it was possible to programme a unit into individual drive modes for different routes you could chop and change between a 153/156/158 style stopper and a 100mph semi fast regional.
could be do-able with EMU/BEMU/DEMU as the gearing is not necessarily fixed,you can just energise/de-energise poles on a traction motor as and when needed,and have pre-selected traction control in the onboard software.
might need different engines for express operation,but theoretically running gear could be standardised, things like drag play a much greater part at speed, so you'd need to look at aerodynamics and power to weight etc....having said that,drag also plays a part on rural traction if you only have a sparcely populated cabin..you need to get as much mpg as possible.
even a motorcyclist on a 125 can tell you you get better mpg/mph cowered behind a faring at 70mph than sitting upright(realistic 10% improvement in both)..I dread to think just how muchcounter- force a 3m*3m flat face of a sprinter has on fuel economy at those speeds.