Originally Posted by PHL10
I think it's probably necessary. For example, there is no point in the El making all its stops if the cars are full and can't take passengers before it even gets to Girard. Then you'll be stuck with stations throughout the system full of passengers that can't board - now that would be a major headache.
That's just the el, though, and the el is, er, unique equipment. This is true for the trolleys -- unique track gauge -- and (to a lesser extent: it might fit the New York subway's A division (IRT) trains) the NHSL as well.
As far as Regional Rail goes there really is no reason why SEPTA can't just borrow some Arrow IIIs for the duration (and possibly some M9s too). The BSL has the same loading gauge as the New York subway's B division (BMT/IND), but I would imagine moving that equipment would be much more of a headache than the mainline stuff.