And by the way there's also #12, which still operates, and until last year did carry passengers. There is also a #10 that does not carry passengers but also runs Washington to Springfield. Both of these are passenger-less mail runs like #13.
I think they could operate as passenger trains, and in these belt-tightening times, they may have to. The loading and unloading of mail cars in places like New York City and Philadelphia should be accomplished during the scheduled station dwell times of many of the Regional trains. Of course the numbers would have to be changed to the 140's to reflect their Springfield-Washington status; chances are they would be combined with existing runs for efficiency.