The Collaborative International Dictionary of English v.0.48:

Ton mile \Ton mile\ (Railroads)
   A unit of measurement of the freight transportation performed
   by a railroad during a given period, usually a year, the
   total of which consists of the sum of the products obtained
   by multiplying the aggregate weight of each shipment in tons
   during the given period by the number of miles for which it
   is carried.
   [Webster 1913 Suppl.]