Thanks for commenting. Sorry, it would be foolish to allow individual private vehicles on the tracks. It would need to be a fleet, of course, probably operated by a single entity but supervised by MTA (in case of NYC). It would need to be able to operate without any changes to the scheduling and signaling of the long trains, because that infrastructure is just not changing quickly. But precisely because the headways are long, and because there are so many interchanges, there is space even during rush hour for a good number of, say, 5 foot long vehicles to schedule themselves in between.
