Work rules governing use of passenger uniforms and specifying which conditions would require passenger, freight, or mixed rates of pay, respectively, were system agreements for one particular railroad, whereas rates of pay (dollar amounts) were in the national agreement.
That's why I inquired about the PRR and NYC practices regarding train crew passenger uniforms for mail and express trains. If they were manned by a passenger pool or protected by a passenger extra board, uniforms might more likely have been required, especially, as MCLAUGHLINNYC stated, there could have been a passenger train in one direction and mail and express in the other. On the other hand, if the mail and express trains were "assigned" trains (where the train crew worked only the assigned train in each direction) I would not be surprised that passenger uniforms were not required.
The passenger rate of pay was lower than the freight rate, and ran 150 miles before per-mile increases kicked in, versus 100 miles for the already higher freight rate. Thus, there is no doubt that the railroad would have desired the mail and express trains to be "passenger" for pay purposes, rather than "freight".
So, I wanted you to know what drove my inquiry, and thank you for all the replies. The one mail and express train that I had experience with was Santa Fe Nos. 3, 4, 7 and 8, the Fast Mail and Express, Santa Fe did not list the mail train in the passenger timetable, but the trains paid passenger rate, and passenger uniforms were required. Express boxcars with communication and steam pipes running through them were considered passenger cars on Santa Fe. The rear car was a "rider" car for the train crew, normally a heavyweight combine or heavyweight chair car, clean, well-maintained, and good for 90 MPH. Deadheading of other crews was done on these trains, so sometimes there were several passengers. If you had a ticket and were willing to walk to (or beyond) the far end of the passenger platform, you could ride these trains, but only from and to their regular stops, which were few. I rode No.3 from Clovis to Belen and the Conductor did not collect my ticket. He said he did not want to do all the paper reporting for one fare. On Santa Fe, passenger crews worked their assign train from their home terminal and first in/first out from the away-from-home terminal on the return trip. Thus, a crew assigned to work the Fast Mail and Express out of their home terminal and the Grand Canyon homeward, might work any passenger train homeward if it came in ahead of a late Grand Canyon. That's the reason the passenger uniforms were required on Santa Fe mail trains.
No waycar (caboose) was ever regularly used on ATSF heavyweight passenger trains, to the best of my knowledge.