An interesting comparison with the previous decade, 1870 to 1880, exemplifies this relation still further. The gross earnings of the trunk lines of the United States decreased very greatly per mile of line from $7,211 in fact to $6,636 during the decade; but at the same time the net earnings steadily increased. This was due primarily to the great volume of business developed,--the ton mileage increasing more than three fold during these ten years. It happened despite the fact that the miles of line during the same period had more than doubled. The following decade, 1880 to 1890, was represented by an increase of only 82.7 per cent. in mileage, while the number of tons of freight hauled one mile increased by 132 per cent. Density increasing in this way, a corresponding ability to carry at a lower rate per ton was a necessary result. So indisputably has this law--that an expanding volume of business up to a certain point, may profitably be carried at a continually lowered cost--been proved, that it is estimated by so competent an authority as the _Engineering Review_ that, provided sufficient tonnage be available for 2,000-ton freight train loads, a cost of one mill per ton mile can be attained. Its significance may be realized from the fact that the lowest revenue per ton mile reported for the United States is 2.21 mills per ton mile for the long haul soft coal business of the Chesapeake & Ohio.[58] This, of course, does not imply that any railroad in actual operation, carrying all kinds of freight including a large proportion of local traffic, can in the immediate future hope to attain this result. It is intended only to show that, provided the volume of traffic be large enough, the cost of operation tends to decline as a matter of course, until a condition of congestion for the existing plant has been reached. At this point a new cycle of costs of operation and of profits makes its appearance.