If anyone is ever able to get to Steamtown National Historic Site in Scranton PA they have an old mail car you can walk thru. Worth the visit, can easily spend an entire day going thru the trains and museums.
The line carries U.S. mail and was one of the first recreational railroads to have its own postmark, and is the last railroad in the United States to have continually offered hand-cancelling and processing of mail. The postal cancellation stamp was issued to the railroad by the U.S. Postal Service in 1961. The locomotive of the Zooliner has a postal mail slot on the side of the cab, and mail boxes are located at the Zoo and Washington Park stations.