A railroad is a land transportation system in which cars with flanged steel wheels run on tracks of two parallel steel rails. From their beginning railroads provided reliable, economical transport for freight and passengers; they promoted the Industrial Revolution and have been vital to continued economic growth ever since, especially in developing countries. Railroads are intrinsically economical because the rolling friction of wheel on rail is very low, so that a locomotive of 750 watts(1 hp) per gross tonne is needed – 10% of that required for road transport. However, fixed costs of maintenance etc., are high, so high traffic volume is needed. This, together with rising competition an overmanning, led to the closure of many minor lines in the US and Europe, though new lines have been elsewhere. Maintenance, signaling, and many other functions are now highly automated.
Railroads developed out of he small mining tracks built in the UK and Europe from the mid-16th century. They used gravity or horse power, and the cars generally ran on flanged rails or plateways. These were hard to switch, however, and the system of flanged wheels on plain rails eventually predominated. The first public freight railroad was the Surrey Iron Railway (1801). The modern era of mechanized traction began with Richard Trevithick's steam locomotive New Castle (1804) (see also steam engine. Early locomotives ran on toothed racks to prevent slipping, but in 1813 this was found to be unnecessary. The first public railroad to use locomotives and to carry passengers was the Stockton to Darlington Railway (1825). The boom began when the Liverpool and Manchester Railway opened in 1830 using George Stephenson's Rocket, a much superior and more reliable locomotive. (See also early history of railways in Britain.) Railroads spread rapidly in Britain, Europe, and the US. The first US railroad was the Baltimore and Ohio (1830).
The rails were laid on wooden (later also concrete) cross ties or sleepers, and were joined by fishplates to allow for thermal expansion. Continuous welded rails are now generally used. Track gauges were at first very varied, but the "standard gauge" of 4 feet 8½ in (1.435 m) soon predominated. Railways must be built with shallow curves and gentle gradients, using bridges. embankments, cuttings, and tunnels as necessary.