The total tons (last column) are obtained by sum of the first four columns and the total row at the bottom is obtained by the sum over all of the carriers (including those not shown in this brief snippet).  The fourth numeric column, tons received from connecting roads is actually what we are after, the total number of tons of pure interchange (or overhead) traffic.  It was my hypothesis that the number of tons in this column would be greater for commodities that would be diverted more frequently.