A train is a form of rail transport consisting of a series of vechiles propelled along a rail track to transport cargoor passangers. Motive power is provided by a separate locomotives or individual motors in self-propelled multiple units. Although historically steam propulsion dominated, the most common modern forms are diesel and electrical locomotives, the latter supplied by overhead wires or additional rails. Other energy sources include horses,ropes or wires ,gravity, pneumatics, battries, and gas turbines. Train tracks usually consists of two, three or four rails, with a limited number of monorailsand maglev guideways in the mix. The word 'train' comes from the old frenchtrahiner, from the latin trahere 'pull, draw'.There are various types of trains that are designed for particular purposes. A train can consist of a combination of one or more locomotives and attached railroad cars or a self-propelled multiple unit (or occasionally a single or articulated powered coach, called a railcar. The first trains were rope-hauled, gravity powered or pulled by horses. From the early 19th century almost all were powered by steam locomotives. From the 1910s onwards the steam locomotives began to be replaced by less labour-intensive and cleaner (but more complex and expensive) diesel locomotives and electrical locomotives, while at about the same time self-propelled multiple unit vehicles of either power system became much more common in passenger service.