I actually think this is the wrong approach. You don't get to reap any benefits until you start eliminating the old system, and with the parallel approach you d

I actually think this is the wrong approach. You don't get to reap any benefits until you start eliminating the old system, and with the parallel approach you defer that cleanup period until the transition is complete. It'd be far better if adding CBTC yielded the cost savings of eliminating the old system now as it rolled out. That requires more complex inter-op systems and more expensive upgrades to trains, but both of those things are not the constraint. The core constraint is the time you can shut down the subways for in order to do track & signal work in place. When you have a critical constraint, you should do lots of extra work in other places to maximize the productivity of the constrained operation.