I know about interlining and trunk lines etc. So about a third of the fleet is incompatible right now. I assume the problem is equally bad in both A division a

I know about interlining and trunk lines etc. So about a third of the fleet is incompatible right now. I assume the problem is equally bad in both A division and B division (fat trains and thin trains for those reading along). The track sharing doesn't seem like a huge deal in my proposal. Services usually don't share track across trunk lines. I can't think of any platforms that are shared by trains on different trunks. As an example you could probably get away with enforcing A/C/E are new cars only. Worst case you delay installing the barriers on the shared track segments. There's also the obvious solution of using the L and G as the system test case (yet again). MTA should take the opportunity while the L is shut down anyway for the Canarsie tunnel repair clusterfudge. You're right that it would reduce operational flexibility a bit. In theory service reroutes should be uncommon enough for this to not matter. But you and I know better. If I were to draft this as a serious proposal I'd look at reroute probabilities, come up with some way to estimate the cost incurred by inflexibility, and optimize the installation of platform walls to balance this cost constraint.