Actually, the ability to handle certain grades is predicated on total train weight (therefore length).
If, after opening the new line, you ascertain that you need to add 1/3 to the length of the train, its not simply tacking on more cars......its discerning whether you may have to tear down entire segments of line and rebuild from scratch.
There's also the matter of station placement; if any of those stations is near a curve on one side, never mind two, extending them may prove impossible, not merely expensive and disruptive.
I'm not firmly opposed to choosing different rolling stock than the TTC standard; or going to standard gauge (though i see little advantage to this and creates problems, like lack of interoperability); but reducing capacity to save a few $$$ now, seems penny-wise, pound foolish.