This talk of speed restrictions on the wires south of Bedford (or Kettering, once the upgrades to Corby are in place) misses an important point - the IEP units will require an additional stop to change power modes to gain the full benefits of electric traction, or otherwise just stick with the diesel power all the way to St. Pancras, and vice-versa on the way up to Sheffield. Even in the most hopeful scenario, that will add a few minutes onto any schedule.
What will happen to the services that currently run non-stop from Leicester to St. Pancras? Changing power modes while still running is seemingly out of the question to my knowledge - it won't be sanctioned to be done at high speeds and no such testing has yet taken place - or are they proposing that drivers just "chance it" if they hit a TSR or amber signal on the wired section going south? If not, you're left with either a completely redundant electric power mode on pretty much all Sheffield and Derby services or an extra stop to facilitate it. Great for folks in Bedford or Kettering, terrible for anybody north of there.
For the record, in line with the current electrification proposals, to reap full benefits with electrification installed to its full extent, a Leeds to London train would have to:
- Run electric-powered Leeds to Wakefield (11 miles)
- Switch to Diesel at Wakefield
- Run Diesel powered Wakefield to Sheffield (25 miles)
- Switch to Electric at Sheffield
- Run electric-powered Sheffield to Chesterfield (15 miles)
- Switch back to Diesel at Chesterfield
- Run Diesel-powered from Chesterfield to Kettering/Bedford (85/102 miles)
- Switch to electric at Kettering or Bedford
- Run Electric-powered Kettering/Bedford to London St. Pancras (71/54 miles
The Sheffield section, for the record, is not due to be electrified until HS2 phase 2 is completed in 2032/33 at a minimum. The IEP units will already be 10 years old by that point, and if the proposed restriction on HS2 access from "classic rail" is anything to go by, there's no guarantee that they'll be able to use the wires anyway.
So under current proposals, a bi-mode unit using electric traction for the maximum possible length of line would utilise its power modes for the following distance on a Leeds-London run:
Diesel - 110 miles
Electric - 97 miles
In other words, even in a best-case scenario which assumes that the IEPs will be allowed to operate on HS2's Sheffield - Chesterfield wires, under current proposals diesel would be the primary power mode, rather than the backup, for the duration of the IEP's MML career. As the likelihood of every bi-mode train stopping at Kettering to switch is low, Bedford will probably be the location for such a transition, making the figures:
Diesel - 127 miles
Electric - 80 miles
TL;DR - The Bi-modes will use diesel as a primary power source for their whole career. They're pointless. You may as well just buy new DEMUs with better acceleration.
(Disclaimer - these are not 100% accurate milepost readings but approximations based on map data.)