Axle, a piece of timber or a bar of iron which supports the body of a car, carriage, or wagon, and is itself supported on two wheels, in the hubs or naves of which its ends are inserted. A great change was introduced about 45 years ago in the shape of axles for carriages, by the English invention of air-tight closed boxes, which with slight modifications has been adopted all over the world. The wheels of carriage axles are prevented from falling out by means of a collar on the axle, which enters the hub on the inside, and not by a nut and pin on the outside, as usual in common vehicles. The introduction of railroads has made another change necessary. Axles for railroads, instead of revolving in the hubs of the wheels, are strongly keyed in them, and journals are turned on the portions outside the wheels. These journals pass through and revolve in boxes attached to the frame of the cars. This arrangement has been found to resist vibrations and jerks resulting from high velocity much better than the old plan. It was, moreover, necessary to insure a distance between the rims of the wheels invariably equal to that of the rails.

It has been attempted to divide axles in the centre, the inner ends of the two half axles being maintained in boxes fixed in slides on a frame, and the body of the carriage acting as a lever on a small mechanism, and bringing each axle perpendicular to the curve of the road. One wheel has also been made to revolve around the axle, which was fixed to the other wheel, and turned with it; in this way railroad cars would turn a short curve without straining the axle. Such arrangements, however, have never been extensively introduced, as the disadvantages from complexity and loss of strength outweigh the advantages gained in turning curves. In horse cars running on city railroads, the difficulty of turning street corners, through curves of very short radius, is simply met by causing the outer wheel to run on its flange on a flat rail; it thus acts as a larger wheel and passes through a greater distance with the same number of revolutions as the inner wheel, and thus describes a curve, notwithstanding the wheels are all immovably connected with the axles.

The difficulty of turning curves is not only in the straining of the axles immovably fixed to the wheels, but also in the rigid parallellism of the forward and rear axles, which opposes the turning of a curve the more in proportion as the car is longer; and as American passenger cars are very long and curves very common, the so-called truck system was adopted, consisting in a frame turning on a vertical axle or pivot, and supported by four or six wheels, of which the axles are parallel. Such a four or six-wheeled frame or truck is placed at each end of the car; and in going around curves the trucks adapt themselves by turning on the central vertical pivot. In Europe, where curves are more avoided regardless of expense, and cars are shorter, this system has not been adopted, except in a few exceptional localities, where curves of short radius could not. be avoided in the construction of the road. The only kind of locomotive where the wheels are not immovably connected with the axle are those lately built for common roads, in some of which the connection is ingeniously made with a gearing, so that notwithstanding both wheels act as driving wheels, they are not compelled to make the same number of revolutions, and thus are able to turn any short curve in a common road.