The capacity is more than adequate for many years to come. And tunnelling was necessary if you wanted to build it. With 90-metre long stations the capacity reduction is primarily in the length of the stations than anything else; which is similar to what we see on the Ontario line as well. If it ever gets to needing 4 times the current anticipated ridership, we'll be into another generation of equipment. At that point a vehicle could be selected that can run automatically every 90 seconds between Pearson and Don Valley station, with minor modifications at Leslie, increasing capacity to heavy rail numbers.
There's certainly modern subways with less capacity. The 40-metres stations (perhaps expandable to 50 metres) of the Canada line come to mind!
Sheppard seems a bit far to me. But there's other alternatives on Lawrence and/or St. Clair - which would be fantastic. But long after most of us are dead, I'd think.
But imagine a St. Clair line, from Warden station to Kipling station. With rail crossings of the ravines and valleys between Mount Pleasant and Woodbine!
