[1] : The Collaborative International Dictionary of English v.0.48

mail \mail\ m=al, 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
see also:
mail catcher mail guard mail train 

[2] : The Collaborative International Dictionary of English v.0.48

mail train \mail train\ n.
   a railroad train that carries mail.
   wordnet 1.5

[3] : WordNet (r) 2.0

mail train
     n : a train that carries mail

Electronic Mail >> E Words
Electronic Mail, definition of term: Electronic Mail
electronic+mail_pag1.html

Commuter Train >> C Words
Commuter Train, definition of term: Commuter Train
commuter+train_pag1.html

Chain Mail >> C Words
Chain Mail, definition of term: Chain Mail
chain+mail_pag1.html

Bulk Mail >> B Words
Bulk Mail, definition of term: Bulk Mail
bulk+mail_pag1.html

Air Mail >> A Words
Air Mail, definition of term: Air Mail
air+mail_pag1.html