\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.