a train consists of a single or several connected rail vehicles that are capable of being moved together along a guide way to transport freight or passengers from one place to another along a planned route. The guideway (permanent way) usually consists of conventional rail tracks, but may be monorail or maglev. Propulsion for the train may come from a variety of sources, but most often from a locomotive or self-propelled multiple unit.  In railway terminology, a consist is used to describe the group of rail vehicles which make up a train.

Types of trains

There are various types of trains designed for particular purposes, see rail transport operations.A train can consist of a combination of a locomotive and attached railroad cars, or a self-propelled multiple unit (or occasionally a single powered coach, called a railcar). Trains can also be hauled by horses, pulled by a cable, or run downhill by gravity.
Special kinds of trains running on corresponding special 'railways' are atmospheric railways, monorails, high-speed railways, maglev, rubber-tired underground, funicular and cog railways.

A passenger train may consist of one or several locomotives, and one or more coaches. Alternatively, a train may consist entirely of passenger carrying coaches, some or all of which are powered as a "multiple unit". In many parts of the world, particularly Japan and Europe, high-speed rail is utilized extensively for passenger travel.

Freight trains comprise wagons or trucks rather than carriages, though some parcel and mail trains (especially Travelling Post Offices) are outwardly more like passenger trains.

In the United Kingdom, a train hauled by two locomotives is said to be "double-headed", but in Canada and the United States, it is quite common for a long freight train to be headed by three, four, or even five locomotives.

Trains can also be mixed, hauling both passengers and freight, see e.g. Transportation in Mauritania. Such mixed trains became rare in many countries, but were commonplace on the first 19th century railroads.

Special trains are also used for track maintenance.

A single uncoupled rail vehicle is not technically a train, but is usually referred to as such for signaling reasons.
Motive power

The first trains were rope-hauled or pulled by horses, but from the early 19th century, almost all were powered by steam locomotives. From the 1920s onwards they began to be replaced by less labor intensive and cleaner (but more expensive) diesel locomotives and electric locomotives, while at about the same time self-propelled multiple unit vehicles of either power system became much more common in passenger service. Most countries had replaced steam locomotives for day-to-day use by the 1970s. A few countries, most notably the People's Republic of China where coal is in cheap and plentiful supply, still use steam locomotives, but this is being gradually phased out. Historical steam trains still run in many other countries, for the leisure and enthusiast market. Electric traction offers a lower cost per mile of train operation but at a very high initial cost, which can only be justified on high traffic lines. Since the cost per mile of construction is much higher, electric traction is less favored on long-distance lines. Electric trains receive their current via overhead lines or through a third rail electric system.

Passenger trains

Passenger trains have passenger cars. Passenger trains travel between stations; the distance between stations may vary from under 1 km to much more.

Long-distance trains, sometimes crossing several countries, may have a dining or restaurant car; they may also have sleeping cars, but not in the case of high-speed rail, these arrive at their destination before the night falls and are in competition with airplanes in speed. Very long distance trains such as those on the Trans-Siberian railway are usually not high-speed.

Very fast trains sometimes tilt, like the Pendolino. Tilting is a system where the passenger cars automatically lean into curves, reducing the centrifugal forces acting on passengers and permitting higher speeds on curves in the track with greater passenger comfort.

For trains connecting cities, we can distinguish inter-city trains, which do not halt at small stations, and trains that serve all stations, usually known as local trains or "stoppers" (and sometimes an intermediate kind, see also limited-stop).

For shorter distances many cities have networks of commuter trains, serving the city and its suburbs. Some carriages may be laid out to have more standing room than seats, or to facilitate the carrying of prams, cycles or wheelchairs. Some countries have some double-decked passenger trains for use in conurbations. Double deck high speed and sleeper trains are becoming more common in Europe.

Passenger trains usually have emergency brake handles (or a "communication cord") that the public can operate. Abuse is punished by a fine.

Large cities often have a metro system, also called underground, subway or tube. The trains are electrically powered, usually by third rail, and their railroads are separate from other traffic, without level crossings. Usually they run in tunnels in the city center and sometimes on elevated structures in the outer parts of the city. They can accelerate and decelerate faster than heavier, long-distance trains.

A light one- or two-car rail vehicle running through the streets is not called a train but a tram, trolley, or streetcar, but the distinction is not strict.

The term light rail is sometimes used for a modern tram, but it may also mean an intermediate form between a tram and a train, similar to metro except that it may have level crossings. These are often protected with crossing gates. They may also be called a trolley.

Maglev trains and monorails represent minor technologies in the train field.

The term rapid transit is used for public transport such as commuter trains, metro and light-rail.

Freight trains

Freight trains have freight cars.  Freight cars transport product such as clothing, footwear, textiles, etc.

Much of the world's freight is transported by train. In the USA the rail system is used mostly for transporting freight (or cargo).

Under the right circumstances, transporting freight by train is highly economic, and also more energy efficient than transporting freight by road. Rail freight is most economic when freight is being carried in bulk and over long distances, but is less suited to short distances and small loads.

The main disadvantage of rail freight is its lack of flexibillity. For this reason, rail has lost much of the freight business to road competition. Many governments are now trying to encourage more freight onto trains, because of the environmental benefits that it would bring.

There are many different types of freight train, which are used to carry many different kinds of freight, with many different types of wagon. One of the most common types on modern railways are container trains, whereby the containers can be lifted on and off the train by cranes and loaded off or onto trucks or ships.

This type of freight train has largely superseded the traditional "box wagon" type of freight train, whereby the cargo had to be loaded or unloaded manually.

In some countries "piggy back" trains are used whereby trucks can drive straight onto the train and drive off again when the end destination is reached. A system like this is used on the Channel Tunnel between England and France. Piggy back trains are the fastest growing type of freight trains in the United States, where they are also known as 'trailer on flat car' or TOFC trains. There are also some "inter-modal" vehicles, which have two sets of wheels, for use in a train, or as the trailer of a road vehicle.

There are also many other types of wagon, such as "low loader" wagons for transporting road vehicles. There are refrigerator wagons for transporting food. There are simple types of open-topped wagons for transporting minerals and bulk material such as coal and tankers for transporting liquids and gases.

Freight trains are sometimes illegally boarded by passengers who do not wish, or do not have the money, to travel by ordinary means. This is referred to as "Hopping" and is considered by some communities to be a viable form of transport. Most hoppers sneak into train yards and stow away in boxcars. More bold hoppers will catch a train "on the fly", that is, as it is moving, leading to occasional fatalities, some of which go unrecorded.