But designing a new class of DMU when we have more than enough of the things already just to serve a few marginal routes is a waste of money. Modify what we have by all means, making voyagers able to operate electrically is a good. If MML was electrified it would not be value for money to keep the Lincoln service.
If I assume you mean intercity DMUs like Voyagers and Class 180s.....
If the Birmingham-North services are converted to electric traction then that frees ten additional Class 221s.
If we assume formations can be reshuffled with the addition of pantograph trailers from the eVoyager project then five of them will be used to replace the remaining HSTs at XC.
Five remaining 221 sets, plus however many sets are cascaded from East Midlands trains.... I will assume you axe all non core routes and thus 27 sets will be available.
You now have:
5 5-car 221s (remaining motor cars will have been retained by XC for capacity enhancement)
6 9-car ED 222s
17 6-car ED 222s
4 5-car ED 222s.
If you order additional transformer cars and disproportionate the Class 222s into nine and 5 car sets..... you can convert the fleet into:
5 5-car ED 221s.
11 9-car ED 222s
1 8-car ED 222
qt 5-car ED 222
Now.... you can send all twelve of the long 8/9-car ED 222s to the Great Western to replace its HSTs for Cornwall services and perhaps for an additional round trip to Carmarthen or the like.
This leaves you with:
5 5-car ED 221s
15 5-car ED 222s
So you have a pair of fleets of 221s and 222s, the 221 fleet would be needed for the GC services that are currently all diesel to go to electrodiesel.
The other would be the only fleet of Electrodiesels available for any other operations, and would not even be sufficient for the non electrified TPE branches, let alone all the other routes that are largely under wires but will have to remain diesel operated.
There just aren't enough electrodiesels even with the IEP.
And this is before we get around to replacing "express" services that will operated significantly over wire but will not be operated by electrics because half of the route is not electrified.
And a 4 car ED 22x of the type I described would have similar fuel and track access costs to a 3/4 car lashup of 175s.... on all diesel diagrams.
Anyway there is a speed limit over the flat crossing and frieght trains get held at the sewer so you logic is yet again flawed by what happens daily.
... That doesn't disprove it causing problems for ECML capacity, it just means that it is currently tolerable, if you add two more periods of disruption each day (one round trip), how much more disruption will there be?
By your argument there is no problem with dragging a train over that crossing every 10 minutes because it will have no more impact on the ECML.
EDIT:
And what was wrong with the 373s?
They had more capacity than anything else GNER could hope to have access to