Because by the end of the 1960s much 'passenger' service was hauling mail. A pair of E7s, 1 baggage car, 3 coaches and twelve mail cars would not be unusual. The cab would be needed to for pick-up and set-off moves for the mail cars.Or so I would surmise.Page 104 of Pennsy Diesel Years V. 3 shows a NH cab that made it all the way to Columbus, Oh., as a "rider car on a mail train".Page 114 of the same book shows a Pennsylvania Ltd. made up of "three Es, four baggage cars of mail and express, two coaches, twelve more cars of mail and express, rider cabin car #4723 {four digit number}, and still another car of mail."This was in 1970 (the caption says Amtrak was a year away), so it was a PC train. I don't know if PRR pioneered this, or if PC originated it. But there it is, documented and with photo evidence.Patrick