A concrete example may aid in making this important principle clear. The new through-freight trunk line built by the Pennsylvania Railroad since 1900, paralleling its old four-track one, represents both in the cost of maintenance and capital charges, a sudden jump in the expense of transporting each ton of freight on _both_ lines, until such time as the new business grows to a point where it can support the new line by itself alone. The relation between increasing returns and density of traffic is well illustrated in this instance. With six tracks in operation nearly all the way from Pittsburg to Philadelphia, the four old tracks are sometimes almost fully utilized for passengers and fast freight. The extraordinary density of traffic appears in the statement that this road in 1911 on 3534 miles of track handled one-third more ton miles than the Union Pacific--by far the most worked of all the western lines--handled on 13,674 miles of track. The two new low-grade Pennsylvania freight tracks are used only for slow traffic; largely coal and westbound steel empties. Not-withstanding the extraordinary density of traffic on this extra two track line, it probably does not meet the fixed charges on cost of construction of the line. Yet the new double track was absolutely necessary, regardless of its profitableness, in order to relieve congestion on the old four tracks. In other words, the demands of the service forced an expenditure which in and of itself was not financially self-supporting. But the profit from the old lines would be sufficiently enhanced to take care of the whole. The bearing of such cases upon the capital needs of the future is obvious. A resolutely conservative policy of finance becomes imperative under such circumstances. In much the same way, the general condition of congestion reached in 1903-'05 on the eastern trunk lines and in the West and South in 1906-'07, manifested mainly in the need for more tracks and terminals, represented the permanent outgrowth of the old plant; and necessitated a readjustment of capital expenses for the purpose of enlargement. Viewed in a large way over a term of years, nearly every expenditure, even the fixed charges which appear constant or independent of the volume of business, thus become in reality imbued with more or less variability.