Text A Light Rail
Light rail or light rail transit（LET) is a particular class of urban and suburban passenger railway that uses equipment and infrastructure that is generally less massive than that used for rapid transit systems，with modern light rail vehicles usually running along the system.
Light rail is the successor term to streetcar，trolley and tram in many locales，although the term is most consistently applied to modern tram or trolley operations employing features more generally associated with metro or subway operations，including exclusive rights-of-way，multiple unit train configuration and signal control of operations.
The term light rail is derived from the British English term light railway long used to distinguish tram operations from steam railway lines，and also from its usually lighter infrastructure.
Light rail systems are almost universally operated by electricity delivered through overhead
lines, though several systems are powered through different means，such as the JFK Airtrain，which uses a standard third rail for its electrical power, and trams in Bordeaux which use a special third-rail configuration in which the rail is only powered while a tram is on top of it. A few unusual systems like the River Line in New Jersey and the O-Train in Ottawa use diesel-powered trains，though this is sometimes intended as an interim measure until the funds to install electric power become available.
轨道交通专业Definition
Most rail technologies，including high-speed，freight，commuter/ regional，and metro/subway are considered to be”heavy rail” in comparison. A few systems such as people movers and personal rapid transit could be considered as even”lighter"，at least in terms of how many passengers are moved per vehicle and the speed at which they travel. Monorails are also considered to be a separate technology. Light rail systems can handle steeper inclines than heavy rail，and curves sharp enough to fit within street intersections. They are generally built in urban areas，providing frequent service with small，light trains or single cars
The most difficult distinction to draw is that between light rail and streetcar or tram systems. There is a significant amount of overlap between the technologies，and it is usual to classify street-cars/trams as a subtype of light rail instead of as a distinct type of transportation. The two common versions are：
1.The traditional type，where the tracks and trains run along the streets and share space with road traffic. Stops tend to be very frequent，but little effort is made to set up special stations. Because space is shared，the tracks are not usually visible.
2. A more modern variation，where the trains tend to run along their own right-of-way and are of- ten separated from road traffic. Stops are usually less frequent，and the vehicles are often got on from a platform.
Tracks are highly visible，and in some cases significant effort is used to keep traffic away through the use of special signaling and even grade crossings with gate arms. At the highest degree of separation，it can be difficult to draw the line between light rail and metros，as in the case of London’s Docklands Light Railway，which would likely not be c
onsidered“light”compared with London Underground.
Many light rail systems have a combination of the two，with both on road and off road sections. In some countries，only the latter is described as light rail. In those places，trams running on mixed right of way are not regarded as light rail，but considered distinctly as streetcars or trams. Light rail is usually powered by electricity，generally 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. In some cases，especially when initial funds are limited, diesel-powered versions have been used，but it is not a preferred option. Some systems，such as the JFK Airtrain in New York City，are automatic without a driver; however, such systems are not what is usually thought of as light rail. Automatic operation is more common in smaller people mover systems than in light rail systems，where the possibility of grade crossings and street running make driverless operation of the latter inappropriate.
Advantages of light rail
Light rail systems are usually cheaper to build than heavy rail，since the infrastructure does not need to be considerable，and tunnels are usually not required as most metro systems. In addition，the ability to handle sharp curves and steep gradients can reduce the amount of work required.
Traditional streetcar systems and also newer light rail systems are used in many cities around the world because they generally can carry a larger number of people than any bus-based public transport system. They are also cleaner, quieter，more comfortable，and in many cases faster than buses. In an emergency，light rail trains are easier to evacuate than monorail or elevated rapid rail trains.
Many modern light rail projects re-use parts of old rail networks，such as abandoned industrial rail lines.
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