Railroads are contrivances for obtaining a surface for the wheels of carriages to roll upon, smoother than the surface of a turnpike road, whether Macadamised or paved. To accomplish this, bars of iron are constructed of a suitable length, and laid upon the road, so that they present upwards a smooth surface; their extremities resting upon large blocks of stone firmly imbedded in the earth, called sleepers. These iron bars, which are called rails, are firmly connected end to end, and extend from sleeper to sleeper along the whole line of road, so as to form one continuous smooth track or line of iron surface, upon which the wheels of the carriage roll. Two parallel tracks of these bars are placed at a distance, corresponding to that of the width of the wheels of the carriage intended to run upon them. The wheels are constructed with a ledge of iron projecting at right angles to the faces of their tires, which as they roll catches the inner surface of the rail, so as to prevent the carriage from slipping off at either side. There are several forms of rails in use, some have a saddle edge to receive the vec of the wheel.
When the surfaces of the tire and the rail are clean, the resistance which they present is extremely small, owing to the hardness of the material of which they are composed, and the smoothness of which its surface is susceptible. Two parallel tracks of rails upon which the wheels of the same carriage roll are called "a single line of railway." In order to enable carriages on such a line moving in contrary directions to pass one another, retiring places called sideings are provided at certain intervals, into which a carriage may be turned, so that one may wait till another passes. This provision is indispensable where
