Just that with this latest tranche of electrification plans, what stock will be allocated seems to be a lot of educated guess work at the moment.
The rolling stock allocations for the current tranche of electrification schemes is probably little more speculative than those in the past. I'm sure that had there been internet forums in the 1980s when the ECML electrification was being developed than there would have been just as much wild speculation about what rolling stock would be allocated to the route. I'm sure we would have seen plenty of discussions along the lines of:
"Will the ECML be getting class 89s and mark 3s? Or class 91s and mark 4s?" "I thought the 225s were destined for use on the WCML first?" "Will the HST mark 3s be rewired as loco hauled stock to work with the class 89s?" "Will the class 89s work with new loco hauled mark 3s? Will they have plug doors?" And then, once the route learning and driver training runs were up and running "The 91s are now working with mark 3s and converted HST power cars as DVTs: I've heard that when the mark 4 carriages are delivered, some 91+mark 3 rakes will be kept for working off the wires".
You've got to bear in mind the timescales involved as well: It is probably six years at least until the MML electrification is completed: Once again returning to the ECML electrification scheme, six years before the wires were switched on to Leeds, the last of the Deltics had only just been withdrawn and APT was still a going concern on the WCML!
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Is there any reason why they didn't go for EMUs rather than loco and stock?
Because multiple unit formations weren't BRs' policy for Intercity rolling stock? The IC225 formations were the final evolution of the electric APT, even if they incorporated a number of different technologies their genesis and concept was still the same. And BR would have followed exactly the same fomula with the Intercity 250 as well in the early nineties; with class 93 locos, mark 5 carriages and a DVT.
Even the TGV derived Eurostars weren't far removed from the idea of a loco and hauled stock push-pull formations, and it is only in recent times that Alstom have developed a distributed traction variant of the TGV design. Long distance EMUs weren't nearly as prevalent an idea in the early nineties.