CDL.TO
Moderator
The Transit City lines will operate at a higher voltage - 750v dc rather than 600v dc. As well, the cars are wider by a foot - 9 and a half feet versus 8 and a half feet. And of course, there is the whole issue of double-ended versus single-ended, and the doors on both sides. And finally, the Transit City cars will have proper couplers and MU capability, whereas the cars for the legacy system will have couplers designed for emergency use only.
So, as we see above, the track gauge issue is almost the least of their concerns in terms of running the equipment together.
And don't forget signalling. The plan has always been for the Eglinton tunnel to use ATO.