historically steam propulsion dominated, the most common modern forms are diesel and electric locomotives, the latter supplied by overhead wires or additional rails. Other energy sources include horses, engine or water-driven rope or wire winch, gravity, pneumatics, batteries, and gas turbines. Train tracks usually consist of two running rails, sometimes supplemented by additional rails such as electric conducting rails and rack rails, with a limited number of monorails and maglev guideways in the mix.The word 'train' comes from the Old French trahiner, from the Latin trahere 'pull, draw'.