The major IRT lines save for the 7 operate mixed fleets of NTT and R62/A equipment, none of which share door positions IIRC. The Eastern Division still uses old rolling stock like R42s from time to time. The last time they tried getting rid of cars prematurely, the R179 order came in late and they were forced to run what cars remained had more aggressively, degrading reliability.

> You could make the test station one of the L train stations that's going to be closed for two years and install them at a super leisurely pace. (The L is basically completely isolated from the rest of the system, and already requires special rolling stock for CBTC. And it's going to be closed for two years for other repairs.)

This actually was a proposal. It was recently killed in favor of bringing elevators to Union Square, a major subway complex that is not ADA accessible. Which is really why they haven't caught on as a serious engineering proposal; the list of things that would be much better value for money is long, such as systemwide ADA accessibility, CBTC signalling, replacement of rolling stock, refurbishment of dilapidated, water-damaged stations, etc. You could easily come up with $50B worth of upgrades, extensions, etc. before platform doors became a priority. And they're not even that feasible.

> There are just a lot more doors in the station than on each train.

Got a source for this? This is intriguing because I've never heard of something like this. But this also introduces the complexity that the right doors have to open at the right time, lest a blind person walk into a gap between trains and die. Which is a lot to ask from an agency that can barely maintain something as mundane as elevators and escalators.