Tom -
By 1966 the Mail and Express trains like 3 and 4 often had new GP-40's, the 3000's, after they started running Flexivans in mail service. Before that, passenger (80 mph) GP-7's and -9's were common. Most of the shuttles between Harmon and 30th Street in 1960 and 1961, when I was working on the Electric Division, had one or two RS-2's or RS-3's. But I remember one night when I was Yard Clerk at Yonkers; Track 6 was blocked with cars and one of those Harmon jobs had to run through the freight yard. I met him at Babcock Place and "piloted" the crew through the yard to GD. They had 2 PA's (4200's)!
Up to the early 1960's, good E-7's and E-8's for passenger service were in short supply, and they didn't often show up on mail trains. But by 1965 there were more E's than passenger trains, so they were more common on mail trains. I can remember No. 23 (Boston-Chicago) at Cleveland many times in 1966 with five E units. Those were long trains, sometimes as many as 40 baggage/mail storage cars, express refrigerators and RPO's.
Up to the late 1950's they would have a caboose for the crew - some had been modified with no cupola and added air signal lines. Then NYC took some surplus commuter coaches, blanked out the windows in the middle of the car and installed oil-fired heaters with a big oil tank in the darkened center of the car. They were used as rider cars on most of the mail trains. When the oil tanks were full they would slosh all over the place, and the darn cars would stink of diesel fuel.