From these comparatively small beginnings, the magnificent equipment of our railroads has grown. From Peter Cooper's locomotive, which weighed less than a ton, with a boiler the size of a flour-barrel, and which had difficulty in beating a gray horse, we now have locomotives which will easily run sixty and can exceed seventy miles an hour, and others which weigh seventy-five tons and over. A comparison of the engraving of Peter Cooper's engine with that of the modern standard express passenger locomotive (Fig. 11) shows vividly the progress which has been made since that first experiment was tried—little more than half a century ago. In that period there have been many modifications in the design of locomotives to adapt them to the changed conditions of the various kinds of traffic of to-day. An express train travelling at a high rate of speed requires a locomotive very different from one which is designed for handling heavy freight trains up steep mountain grades. A special class of engines is built for light trains making frequent stops, as on the elevated railroads in New York, and those provided for suburban traffic (Fig. 12)—and still others for street railroads (Fig. 13), for switching cars at stations (Fig. 14), etc. [[Pp. 110] and [113]]. The process of differentiation has gone on until there are now as many different kinds of these machines as there are breeds of dogs or horses.
Fig. 11.—A Typical American Passenger Locomotive.
Fig. 12.—Locomotive for Suburban Traffic. By the Baldwin Locomotive Works, Philadelphia.
Fig. 13.—Locomotive for Street Railway. By the Baldwin Locomotive Works.
Nearly all the early locomotives had only four wheels. In some cases one pair alone was used to drive the engine, and in others the two pairs were coupled together, so that the adhesion of all four could be utilized to draw loads. The four-wheeled type is still used a great deal for moving cars at stations, and other purposes where the speed is comparatively slow. But to run around sharp curves the wheels of such engines must be placed near together, just as they are under an ordinary street-car. This makes the wheel-base very short, and such engines are therefore very unsteady at high speeds, so that they are unsuited for any excepting slow service. They have the advantage, though, that the whole weight of the machine may be carried on the driving-wheels, and can thus be useful for increasing their friction, or adhesion to the rails. This gives such engines an advantage for starting and moving heavy trains, at stations or elsewhere, which is the kind of service in which they are usually employed.