AFAIK they won't work on any other subway system in the country - no one else uses our loading gauge, and generally no two subways in the country are really compatible with each other rolling-stock wise. I believe one of contributing factors to the full fleet replacement was also that the cost wasn't that much higher than the need midlife rebuild/overhauls which were overdue (and thus never done as the order was expanded).
Actually, it was cheaper to buy new than overhauling the #3 cars to similar specifications as the #4s.
Isn't the Orange Line compatible with PATH trains? Maybe NJ DOT will latch on to this procurement at some point.
Nope; not even close. I know there are some rumblings among the railfan community that the Hawker-Siddeley Orange and Blue Line cars were based off of the PA-3 design. That may have been true on a very fundamental level, but the cars are otherwise very different, right down to the throttle controller handle.