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.