According to the notes in the left bottom corner of the map, the "Figures shown are 1963 gross tonnages, in hundred thousands, arrows indicate direction of movement ...".
In some
other similar maps that Trains has published, they have explicitly stated that gross tonnages include empties. For example ...
Quote
Trains.com
Shown here are the traffic densities on the busiest U.S. main lines in the early '70s. The lightest routes depicted are those carrying 10 million gross tons per year. (Gross tonnage includes the weight of locomotives, cabooses, loaded commodities, and empty cars.)
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trn.trains.com]
Edited 2 time(s). Last edit at 12/04/2018 10:42AM by Rader Sidetrack.