The interrelation between these various factors may be more readily shown by confining our attention to the period during which a practically uninterrupted development of business ensued, thus eliminating the confusion due to the four years of depression after 1893. The data on our various charts for the years 1898-1906 demonstrate that during this period the ton mileage, measuring the freight traffic handled, has practically doubled. To transport this doubled tonnage, a growth in freight train mileage of only eighteen per cent. was necessary. This was due, of course, to the notable concentration of train loading, already described, as well as to a density of traffic per mile of line almost sixty per cent. greater. As a consequence of these economies in operation, the revenue per freight train mile has increased by about fifty per cent.; while the average cost of running all trains per mile has grown less rapidly, namely, by 42 per cent. Had we data for freight trains alone it would surely be lower than this. In the meantime during this period of eight years, the rate of return in revenue per ton mile received, remained practically unchanged.[63] From all of which it would appear that even despite all these confusing factors, the law of increasing returns, so far at least as 1898-'06 was concerned, was making itself appreciably felt. Attentive consideration of the available figures, especially as shown by diagram on page 97, shows apparently that the various economies in operation, heavier trainloads and the like, have not since 1906 yielded any greater profit from mere operation, with the ever increasing volume of business. In other words, the increase in the margin between cost of operation and revenue per train mile,--measuring profitableness per unit of movement--has not kept pace with the augmentation of the size of that unit,--the trainload. Thus it follows, as one would expect, even making allowance for all changes in rates, wages and other expenses, that the law of increasing returns as applied to railroads, does not arise primarily from economic considerations as to mere physical operation. The law originates primarily in the fiscal conditions attaching to the heavy capital investment,--the fact, namely, that fixed charges up to a given point of saturation tend to remain constant, absolutely; but become proportionately less, therefore, as the volume of business expands. From this fact, therefore, rather than because of any marked economies of large-scale production, may it be affirmed that railroads offer a notable example of the law of increasing returns. The important bearing of this distinction will appear in due time in connection with the problem of the determination of reasonable rates. Added significance, also, is given to the relation between the cost of new capital, measured by the rates of interest on bonds and dividends on stocks, and the supply necessary to provide adequate extensions and improvements in future.