=351. Importance of steel in railroad construction.=—To many readers practical devices like the air-brake, further reducing the cost and increasing the efficiency of operation, will be familiar. One factor in improvement, however, is not so apparent, and deserves special attention by reason of its commanding importance. The railroad in its modern form would be impossible if Bessemer and others had not taught the world to make steel cheaply. Iron rails, even under comparatively light loads, wore out and had to be replaced constantly. Steel rails, introduced gradually after 1860, could bear double the load on each wheel, and still outlive many iron rails. The modern rail, simple as it appears, is both in material and in proportions a great feat of engineering, “a beam whose every dimension and curve and angle are exactly suited to the tremendous work it has to do.” Steel rails and steel bridges have made possible the economy of the colossal locomotives of modern times. Steel has enabled men, instead of building 10-ton cars to carry 10 tons of cargo, to build 12-ton cars to carry 20, or 14-ton to carry 30; each improvement of this kind represents a saving in the dead weight of the train, and a consequent reduction in cost. Steel has furnished a material for the bridges over which the cars are carried, enabling a span of 500 feet to be constructed as readily as a span of 250 feet, with the iron formerly employed. [Illustration: GROWTH OF THE EUROPEAN RAILROADS BORMAY ENG. CO. The size of the small circles indicates the railroad mileage of each country at ten year intervals. To facilitate comparison, the circles for 1850 are printed black, and those for 1890 are shaded. Note the disproportion of mileage and area. ]