Commuter rail ought to optimize for seating and ride comfort on long journeys with infrequent stops. Subways ought to optimize for capacity and efficiency on short, mostly standing-room trips within densely populated areas. Unification with subway rolling stock would be awful for commuter rail riders’ comfort and willingness to use the system. Unification with commuter rail rolling stock would be awful for subway crowding.
No one is suggesting unifying subways and commuter rail. Note that LIRR, Metro North, and NJ Transit are all commuter rail. (FWIW commuter rail is a bad term we use here in the states - something like “regional” rail would make more sense.)
