Typically, light-rail systems are built in urban areas, and use short and light trains, (often called trams or streetcars) and can handle steeper gradients and sharper curves than heavy rail.
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.
Terminology is not quite fixed: 'light rail' can be used as a category between tram and heavy rail, but a modern tram system may also be called light rail.
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-busses, they were the only practical means of public transport around cities.
Light-rail systems are still used in many cities around the world, because they have a higher capacity than, and 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 busses.
Many light-rail projects re-use parts of old rail networks, such as abandoned industrial 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 will first be applied on the RijnGouweLijn, to be constructed.
Some of the issues involved are:
- compatibility of the safety systems
- power supply of the track in relation to the power used by the vehicles (voltage, and third rail vs. overhead wires)
- width of the vehicles in relation to the position of the platforms
- height of the platforms