In NYC, the MTA runs both so they are technically consolidated, but changing car widths on a live subway system that is used by millions of people every day is

In NYC, the MTA runs both so they are technically consolidated, but changing car widths on a live subway system that is used by millions of people every day is on the list of very difficult problems. For example, if you wanted to conform the IRT lines in NYC (the numbered lines + the 42nd Street Shuttle) to the BMT/IND lines, you would need to (at minimum), update every platform (to back the edge away from the train), probably update every place two tracks are parallel and too close together, probably also update some of the routing and tunnels to enable a larger turning radius, and reapprove all of the tracks (including elevated portions) to support heavier trains. Since all of the lines (barring the 7 and S) overlap at different points, you can't just make these mutually exclusive changes to one line at a time; you would have to shut down the whole system to make the changes, eliminating service to huge areas of the city for months (if not years).