A train is a connected series of vehicles that move along a permanent track to transport freight or passengers from one place to another. Propulsion for the train is provided by a separate locomotive, or from individual motors in self-propelled multiple units. Most modern trains are powered by diesel locomotives or by electricity supplied by overhead wires or additional rails, although historically (from the early 19th century to the mid-20th century) the steam locomotive was the dominant form of locomotive power.
Rail transport is the conveyance of passengers and goods by means of wheeled vehicles specially designed to run along railways or railroads. Rail transport is part of the logistics chain, which facilitates international trade and economic growth in most countries.
Typical railway tracks consist of two parallel rails, normally made of steel, secured to crossbeams, termed sleepers (UK and Australia) or ties (US). The sleepers maintain a constant distance between the two rails; a measurement known as the "gauge" of the track. To maintain the alignment of the track it is either laid on a bed of ballast or else secured to a solid concrete foundation. The whole is referred to as permanent way (UK and Australia usage) or right-of-way (North American usage).
Railway rolling stock, which is fitted with metal wheels, moves with low frictional resistance when compared to road vehicles. On the other hand, locomotives and powered cars normally rely on the point of contact of the wheel with the rail for traction and adhesion (the part of the transmitted axle load that makes the wheel "adhere" to the smooth rail). While this is usually sufficient under normal dry rail conditions, adhesion can be reduced or even lost through the presence of unwanted material on the rail surface, such as moisture, grease, ice, or dead leaves.
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