Today I pulled out my copy of Two Feet to Tidewater. (Expanded and Updated Edition)
Even the cover photo which shows an engineer, aside from his overalls, is wearing a white shirt and tie!
Railroading used to be a very respected job and folks dressed to show their status.
On Page 61 the gentleman standing near the rear coach in the center photo is wearing a full conductor's uniform with cap, vest and saque coat. (a.k.a "sack" coat)
On page 123 the mail car attendant is also wearing a full uniform with cap, suit and vest. You can bet if the mail agent is in uniform....the rest of the train crew is as well. I can see on a "mixed" that the crew would be wearing bibs, but even there a white shirt and tie and uniform cap would be required. Even in the last days of the line, those in contact with the public dressed relatively formally. On page 209 there is a photo of mail clerk Orvis Bonney with a white shirt and tie.
Most of the photos you see were taken of the locomotives, so the people in those photos would primarily be engine crews who would be attired for their line of work.
Keith