Mail car

Like baggage cars, railway post office (RPO) cars or travelling post offices (TPOs) were not accessible to paying passengers. These cars' interiors were designed with sorting facilities that were often seen and used in conventional post offices around the world. The RPO is where mail was sorted while the train was en route. Because these cars carried mail, which often included valuables or quantities of cash and checks, the RPO staff (who were employed by the postal service and not the railroad) were the only train crews allowed to carry guns. The RPO cars were normally placed in a passenger train between the train's motive power and baggage cars, further inhibiting their access by passengers. [Passenger car (rail). Wikipedia]