A locomotive is a railway vehicle that provides the motive power for a train, and has no payload capacity of its own, its sole purpose is to move the train along the tracks. Many trains feature self-propelled payload-carrying vehicles, these are not normally considered locomotives, and may be referred to as multiple units or railcars (railcar here refers to non-passenger railway vehicles with power to transport personnel and material), the use of these self-propelled vehicles is increasingly common for passenger trains, but very rare for freight. Vehicles, which provide the motive power to haul an unpowered train, are not generally considered locomotives but known as power cars because they have payload space or are rarely detached from their trains.
Introduction of Locomotive
机车是为列车提供动力的铁路车辆，其自身没有运输能力，它唯一的目的是沿轨道 移动列车。许多列车自带动力且有运输能力，这些列车通常不被看作机车，而被认为是动车组或者轨道车辆（轨道车辆在这里指的是用来运送工作人员和材料的具有动力的非 载客用轨道车辆）。这种自带动力的车辆在客运列车中用得越来越多，但货运列车很少被 使用。为无动力的列车提供动力的车之所以不被看作机车，而被看作动力车辆是因为有 运输空间且很少与列车分离。
Traditionally, locomotives haul their trains. Increasingly common these days in passenger service is push-pull operation, where the locomotives push the trains in one direction, and are controlled from a control cab at the opposite end of the train. The locomotives are often subdivided into three main categories according to their usage in rail transport operations. They are passenger locomotives, freight locomotives and switcher (or shunter) locomotives. These categories mainly depend on manoeuvrability, traction power and speed. Some locomotives are designed to work in mountain railways.
长期以来，机车牵引列车运行是普遍方式。而在客运服务中推一拉运行也越来越普 遍，即在列车一端的司机室中控制列车另一端的机车推动列车。按运输用途，机车可以 分为三大类：客运机车、货运机车和调车机车，这些类型主要依赖于机动性，牵引动力 和速度。一些机车被设计用于山区作业。
Locomotives may generate mechanical work from fuel, or they may get power from an outside source. It is common to classify locomotives by the means of providing motive work —one common classification is: steam locomotives, diesel-mechanical locomotives, diesel-electric locomotives, diesel-hydraulic locomotives, gas turbine locomotives, electric locomotives, electro-diesel locomotives, magnetic levitation locomotives. This diagram (Fig.2.1) shows a simplified schematic for electric locomotive power supply system.
The traditional train comprises a collection of coaches (or freight wagons) with suitable motive power attached in the form of a locomotive. The train is made up of sufficient vehicles to carry the traffic offering and is provided with enough power for the job. For passenger operations, one locomotive is usually sufficient. For heavy freight operations, this number might go up to four locomotives on the front and at some other places along the train.
传统的列车由一组客车（或货车）与一节提供相应类型动力的某种型号的机车组成。 列车包括足够的动力和足以完成运输任务的车辆。对旅客运输来说，通常一个机车就足 够了。重载货运时，在列车的前部和其他部位可能达到连挂四个机车。
A good deal of flexibility is possible with locomotive haulage. As long as the train weight remains within the capacity of the locomotive(s), any number of vehicles can be attached, although limitations will be imposed by platform or siding lengths. Locomotives themselves can also be flexible, many of them are designed to cover a range of duties.
机车拖动具有足够的灵活性，只要在机车允许的容量范围内，列车可以编挂任何数 量的车辆，但是列车长度受限于站台或侧线长度。机车自身也是很灵活的，许多机车被 设计用来完成一系列的任务。
The advantages of locomotive hauled trains show they are the best option for many railway operators around the world, particularly freight but, where traffic is dense, i. e” where a large number of trains are required, a more rational approach is necessary, particularly at terminals. In addition, in very predictable operations like commuter services or metro lines, fixed formation trains will be the most efficient.