The Collaborative International Dictionary of English v.0.48:

Mail \Mail\ (m[=a]l), n. [OE. male bag, OF. male, F. malle bag,
   trunk, mail, OHG. malaha, malha, wallet; akin to D. maal,
   male; cf. Gael. & Ir. mala, Gr. molgo`s hide, skin.]
   1. A bag; a wallet. [Obs.] --Chaucer.
      [1913 Webster]

   2. The bag or bags with the letters, papers, or other matter
      contained therein, conveyed under public authority from
      one post office to another; the whole system of appliances
      used by government in the conveyance and delivery of mail
      matter.
      [1913 Webster]

            There is a mail come in to-day, with letters dated
            Hague.                                --Tatler.
      [1913 Webster]

   3. That which comes in the mail; letters, etc., received
      through the post office.
      [1913 Webster]

   4. A trunk, box, or bag, in which clothing, etc., may be
      carried. [Obs.] --Sir W. Scott.
      [1913 Webster]

   Mail catcher, an iron rod, or other contrivance, attached
      to a railroad car for catching a mail bag while the train
      is in motion.

   Mail guard, an officer whose duty it is to guard the public
      mails. [Eng.]

   Mail train, a railroad train carrying the mail.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

mail train \mail train\ n.
   A railroad train that carries mail.
   [WordNet 1.5]

WordNet (r) 3.0 (2006):

mail train
    n 1: a train that carries mail