In response to something I saw earlier about reusing TBMs, we could use the same set of TBMs for 16 years if we build subway extensions in the following order:
1) B-D Extension to Scarborough Centre (2015-2019)
-Constructed
2) Sheppard West Subway extension to Downsview (2019-2023)
-Keeps TBM in-use and adds to the network while DRL is being constructed.
3) Yonge Extension to Richmond Hill (2023-2027)
-Would need to wait for construction of Eastern DRL to be complete to go into service to not overload the system.
4) Sheppard East Subway Extension to Scarborough Centre (2027-2031)
-By this point, demand built up through use of BRT on Sheppard will warrant the construction of a subway connection between Scarborough Centre and Don Mills. B-D Trains and Sheppard Trains could then be interlined
I've illustrated this in this Google Map:
http://goo.gl/maps/BD0x1
The DRL, Eglinton Crosstown, and other projects would be constructed independently with their own TBMs due to significant technology differences between them. The DRL is a far more complicated project and would need to be constructed simultaneously regardless.