The most important single factor in the production of increasing returns upon a railroad is the density of traffic; that is to say, the amount of business which can be conducted with a given set of rails, terminals and rolling stock. In other words, it is the degree of effective utilization of the plant and equipment. It is too obvious to need demonstration, after what has been set forth concerning the nature of railroad expenditures, that economy of operation and, consequently, profits are more or less directly dependent upon this fact. Such effective utilization of the property may be secured in two ways: either by a large tonnage per mile of its line, or else by a concentration of such traffic as it may have into large train loads, which can individually be transported at low cost. The first of these economizes the fixed expenses for roadway and line which respond but slowly to enlargement of traffic, by distributing them thinly over a large tonnage; the second economizes the mere movement expenses which tend to grow less rapidly than the size of the trains. For neither fuel consumption nor wages of train crews expands _pari passu_ with the paying load. Fortunate the lot of the railroad which enjoys both these advantages, of density of traffic per mile of line and of tonnage capable of such concentration in heavy train units. Traffic density--the tons of freight carried one mile per mile of line--is readily computed. The ton mileage, representing the total transportation service, is merely divided by the number of miles of line operated. The following graded table illustrates the wide range of this figure, according to the location of different companies and the nature of their business, as well as the change in the last few years.[59]