The Collaborative International Dictionary of English v.0.48:

mail car \mail car\ n.
   A railway car of special design used for the transportation
   and sorting of mail en route to its destination, and having
   employees of the post office inside to perform those
   functions.
   [WordNet 1.5 +PJC]

WordNet (r) 3.0 (2006):

mail car
    n 1: a railway car in which mail is transported and sorted