The Collaborative International Dictionary of English v.0.48:

Traffic mile \Traf"fic mile\ (Railroad Accounting)
   Any unit of the total obtained by adding the passenger miles
   and ton miles in a railroad's transportation for a given
   period; -- a term and practice of restricted or erroneous
   usage.

         Traffic mile is a term designed to furnish an excuse
         for the erroneous practice of adding together two
         things (ton miles and passenger miles) which, being of
         different kinds, cannot properly be added. --Hadley.
   [Webster 1913 Suppl.]