There really is no such thing as a mail train in the traditional sense (or a freight conductor for that matter), MHC cars are basically mail storage cars (i.e. no sorting, just bags of mail being moved) that run in Amtrak trains. I see the Empire Builder once in a while on my way to work, it often has a couple of MHC’s in it’s consist. Depending on which train it is and how the switching works for it, they can be at the front of the train, but are often at the rear so that they can removed more easily by a switcher when they get to the city they’re going to (which may or may not be the final destination city of the train they’re a part of).
MHCs run in passenger consists. Never as a solid train, although it might be possible on the Northeast Corridor. MHCs carried sealled US Mail and other Express traffic.
Amtrak doesn’t use the MHCs much anymore because they lost their contract with USPS. Now if you see anything besides a baggage or passenger car in an Amtrak train it’s usually an Express Reefer, which look a lot like MHCs.
MHC are not used anymore today in regular service (i think most of the cars have been sold or are sitting idle right now) Amtrak also doesn’t run roadrailers anymore from what i understand, those also went away with the USPS contract. The only thing they still run are the express box cars and those are always run at the rear of the train due to the fact that they do not have HEP which the MHC had on them.