Traffic density has enormously increased during the last two decades, as a result of the filling up of the country and the relative cessation of new construction. This is manifested by the growth since 1890. In that year the density was less than 500,000 ton miles per mile of line, and during the depression of 1893 it fell well below that figure. The total of 1,053,000 reported for 1911 represents, therefore, more than a doubling of the density in twenty years. This growth during 1898 and 1905-'06 was notable. The latter period, especially, was a time when congestion upon all the roads of the country occasioned much distress. The fact is evident that the country has well grown up to the measure of its existing transportation facilities. The second measure of effective operation for the production of increasing returns, is concentration in the trainload. This is regarded by many as the supreme test of efficiency in management. Great progress has been made during the past years in this regard in the United States--an improvement which has largely enabled the carriers to bear up under an increasing burden of expenditure. The trainload is generally adopted today as the unit of operation, measuring the cost of service.[60] It is a fact that, within certain limits, the cost of handling a train does not vary greatly with its capacity. Since the first application of air-brakes to freight trains in 1887, a train crew sufficient to handle fifteen cars can care for thirty about as well in long haul wholesale business. Fuel cost, also, as has been shown, lags well behind the rate of increase of the load. Eaton in his Railroad Operations, concludes that from thirty to fifty per cent. of cost is independent of the trainload. The effect is that any increment in the paying load very materially decreases the cost of operation per ton. Progress in the United States in increasing the average train load is shown by the lowest curve on the diagram on page 97. The scale applicable is along the left hand side of the chart. From 175 tons per train in 1890 to an average figure of 383.10 in 1911 is certainly a remarkable showing. The most rapid increase seems to have occurred after 1897, with the first resumption of general prosperity. As for individual roads, the following table of average train loads is suggestive, as showing the gradation between roads of different type, as well as progress from year to year: AVERAGE NUMBER OF TONS OF FREIGHT PER TRAIN (TONS PER TRAIN MILE)