Light rail is a particular class of railway that includes trolleys and trams as well as modern multi-car trains that operate at street level. In the context of light rail, regular freight, commuter and longer distance railways are called heavy rail (but see also that article for a different usage of that word).
Light rail systems can handle steeper gradients than heavy rail, and curves sharp enough to fit within street intersections. They are typically built in urban areas, providing a frequent service with small, light trains or single cars.
There are two general types of light-rail system. Firstly there is the traditional type where the tracks and trains run along the streets, and share space with road traffic. And secondly there is the type where the trains run along their own right-of-way and are separated from road traffic. There are also many light-rail systems which have a combination of the two, with both on road and off road sections.
They are generally powered by electricity, usually by means of overhead wires, but sometimes by a live rail, also called third rail (a high voltage bar alongside the track), requiring safety measures and warnings to the public not to touch it. Some systems are automatic, dispensing with the need for a driver.
The light rail systems constructed in the late 19th Century and early 20th Century typically included single cars called streetcars, trolley cars, or trams. Light rail lines were also built over longer distances (typically with a single track) before good roads were common; these were generally called interurbans in North America, or radial railways in Ontario.
In North America, many of these original light rail systems were decommissioned in the 1950s and onward as the popularity of the automobile increased. Although some traditional trolley or tram systems still exist to this day, the term "light rail" has come to mean a different type of rail system. Beginning in the 1980s, some cities began reintroducing light rail systems that are more like subway or metro train systems that operate at street level. These light rail system includes modern, multi-car trains that can only be accessed at stations that are spaced anywhere from a couple blocks to a mile or more apart. Some of these systems operate within roadways alongside automobile traffic, and others operate on their own separate right-of-way.
Light-rail systems are generally cheaper to build than heavy rail, since the infrastructure does not need to be as substantial. Moreover, the ability to handle sharp curves and steep gradients can reduce the scale of building work required.
From the mid 19th century onwards, horse-drawn trams were used in many cities around the world. In around the 1880s electrically driven street railways became technically feasible. They became popular because roads were then poorly surfaced, and before the invention of the internal combustion engine and the advent of motor-buses, they were the only practical means of public transport around cities.
Traditional streetcar systems as well as newer light-rail systems are used in many cities around the world because they typically can carry a larger number of people than any bus based public transport system. They are also cleaner quiter, more comfortable, and in many cases faster than buses.
Many modern light-rail projects re-use parts of old rail networks, such as abandoned industrial rail lines.
A good example of both points above is the Docklands Light Railway in London, which uses a sharp, steep, curve to enable it to transfer from running alongside an existing railway line to a disused railway line which crossed underneath the first line. A direct connection between these lines would not be practical for conventional rail.
Around Karlsruhe and Saarbrücken, Germany, light rail vehicles partly use heavy rail tracks, sharing these tracks with heavy rail trains. In the Netherlands this was first applied on the RijnGouweLijn.
Some of the issues involved are: