Expanding Reading（拓展阅读）
The Railroad Track
Rail tracks are used on railroads，which，together with railroad switches，guide trains without the need for steering.Tracks consist of two parallel steel rails，which are laid upon cross ties that are embedded in ballast to form the railroad track.The rail is fastened to the ties with rail spikes or lag screws.
The type of fastener depends partly on the type of sleeper，with spikes being used on wooden sleepers，and screws being used more on concrete sleepers.
Rail tracks are normally laid on a bed of coarse stone chippings known as ballast，which combines resilience，flexibility，and good drainage.Steel rails can also be laid onto a concrete slab.
Tracks have three main functions.It must support the load，provide a smooth surface for easy movement and guide the wheels of the train.
The railroad line should be as level and straight as can be achieved because slopes and curves increase the burden on the locomotive and the wear on the track.
Road bed is the subgrade on which laid the ballast，ties and rails.It should be firm，welldrained and of adequate dimensions.
As a train enters a curve，its natural tendency is to continue going straight ahead.It turns only because the outside rail forces it to do so.To permit trains to traverse curve with safety and greater smoothness，the outer rail is raised above the height of the inner rail so as to balance the forces when the movement of the train is diverted from a straight line to a curve.