Anyone who will stand close to a line of railroad when a train is rushing by at a speed of forty, fifty, or sixty miles an hour must wonder how the engine and cars are kept on the track; and even those familiar with the construction of railroad machinery often express astonishment that the flanges of the wheels, which are merely projecting ribs about 1-1/8 inches deep and 1¼ inches thick, are sufficient to resist the impetus and swaying of a locomotive or car at full speed. The problem of the manufacture of wheels which will resist this wear, and will not break, has occupied a great deal of the attention of railroad managers and manufacturers. [Illustration: Fig. 47. Fig. 48. Fig. 49. Janney Car Coupler, showing the Process of Coupling.] Locomotive driving-wheels in this country are always made of cast-iron, with steel tires which are heated and put on the wheels and then cooled. They are thus contracted and "shrunk" on the wheel. The tread, that is, the surface which bears on the rail, and the flange of the tire are then turned off in a lathe, shown in Figure 25, on p. 121, made especially for the purpose. For engine-truck, tender, and car-wheels, until within a few years, "chilled" cast-iron wheels have been used almost exclusively on American railroads. If the tread and flange of a wheel were made of ordinary cast-iron they would soon be worn out in service, as such iron has ordinarily little capacity for resisting the wear to which wheels are subjected. Some cast-iron, however, has the singular property which causes it to assume a peculiar, hard crystalline form if, when it is melted, it is allowed to cool and solidify in contact with a cold iron mould. The iron which is thus cooled quickly, or "chilled," becomes very hard, and resists wear very much better than iron which is not chilled. Car-wheels which are made of this material are therefore cast in what is called a chill-mould. Figure 50 represents a section of such a mould and flask in which wheels are cast. [Illustration: Fig. 50.--Mould and Flask in which Wheels are Cast.]