I had considered this, but had decided against it because actually upgrading from one type of electrification to another in the game would be very awkward indeed, as the upgraded units would not be able to travel on the same track as the non-upgraded units. This would mean that upgraded units would be able to enter the depot for upgrading, but would get stuck when they try to leave until the power was changed over to overhead AC, which could only be done after all of the units on the line in question had been replaced or modified.
Do you think this sort of upgrade process workable for ordinary players in practice? I can see it causing much confusion. I should be interested in others' views on the topic.
Incidentally, this would require new graphics, not just new .dat files, as the converted units changed in appearance.