By infrastructure, I mean the capacity of the electrification system. If the substation has (and I'm massively simplifying the numbers here for easy of reading by others and because I can't remember the numbers of the top of my head as I'm at work at the moment) a capacity of 2A, and the locomotive draws 1A to run at 90mph, that's all fine and dandy. However, if the service becomes more intensive, and you start to have trains pass each other in the same segment, then you are already drawing the capacity of the substation.
If you are then intent on using a new locomotive that is drawing 1.5A to run at 110mph, then suddenly you can't have two locomotives on the same substation, and the whole electrical infrastructure needs improving.
I agree that the ECML as a whole (and especially York to Darlington) is a fast route; electric trains would inevitably lead to faster times as they are quicker to accelerate, decelerate, and require less energy to run as they are lighter.
Completely agree; the patchwork of schemes and changing systems recommended by the Govt were unfortunate (but sadly understandable)!