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The Collaborative International Dictionary
Traffic mile

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.