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Form of passenger urban rail transit

Light rail transit (LRT) is a form of passenger urban rail transit characterized by a combination of tram and rapid transit features. While its rolling stock is similar to that of a traditional tram, it operates at a higher capacity and speed and often on an exclusive right of way. In many cities, light rail transit systems more closely resemble, and are therefore indistinguishable from, traditional underground or at grade subways and heavy rail metros. There is no standard definition, but in the United States (where the terminology was devised in the 1970s from the engineering term light railway), light rail operates primarily along exclusive rights of way and uses either individual tramcars or multiple units coupled to form a train that has a lower capacity and speed than a long heavy rail passenger train or rapid transit system. A few light rail networks tend to have characteristics closer to rapid transit or even commuter rail; some of these heavier rapid transit like systems are referred to as light metros. Other light rail networks are tram like and partially operate on streets.

The world's first electric tram line operated in Sestroretsk near Saint Petersburg, Russia, invented and tested by Fyodor Pirotsky in 1880. The second line was the Gross Lichterfelde tramway in Lichterfelde near Berlin in Germany, which opened in 1881. It was built by Werner von Siemens who contacted Pirotsky. This was the world's first commercially successful electric tram. It initially drew current from the rails, with overhead wire being installed in 1883. The first interurban to emerge in the United States was the Newark and Granville Street Railway in Ohio, which opened in 1889.

Many original tram and streetcar systems in the United Kingdom, United States, and elsewhere were decommissioned starting in the 1950s as subsidies for the car increased. Britain abandoned its tram systems, except for Blackpool, with the closure of Glasgow Corporation Tramways (one of the largest in Europe) in 1962.

Due to varying definitions, it is hard to distinguish between what is called light rail, and other forms of urban and commuter rail. A system described as a light rail in one city may be considered to be a streetcar or tram system in another. Conversely, some lines that are called "light rail" are very similar to rapid transit; in recent years, new terms such as light metro have been used to describe these medium capacity systems. Some "light rail" systems, such as Sprinter, bear little similarity to urban rail, and could alternatively be classified as commuter rail or even inter city rail. In the United States, "light rail" has become a catch all term to describe a wide variety of passenger rail systems. There is a significant difference in cost between these different classes of light rail transit. Tram like systems are often less expensive than metro like systems by a factor of two or more.

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, many of the same vehicles can be used for either, and it is common to classify streetcars or trams as a subcategory of light rail rather than as a distinct type of transportation. The two general versions are:
The traditional type, where 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 usually visually unobtrusive. A more modern variation, where the trains tend to run along with their own right of way, separated from road traffic. Stops are generally less frequent, and the vehicles are often boarded from a platform. Tracks are highly visible, and in some cases, significant effort is expended to keep traffic away through the use of special signaling, level crossings with gate arms, or even a complete separation (semi metro) with non level crossings.

At the highest degree of separation, it can be difficult to draw the line between light rail and metros. The London Docklands Light Railway would likely not be considered as "light rail" were it not for the contrast between it and the rapid transit London Underground. In Europe and Asia, the term light rail is increasingly used to describe any rapid transit system with a fairly low frequency or short trains compared to heavier mass rapid systems such as the London Underground or Singapore's Mass Rapid Transit. However, upon closer inspection, these systems are better classified as light metro or people movers. For instance, Line 1 and Line 3 in Manila are often referred to as "light rail", despite being fully segregated, mostly elevated railways. This phenomenon is quite common in East Asian cities, where elevated metro lines in Shanghai, Wuhan, and Dalian in China; and Jakarta, Greater Jakarta and Palembang in Indonesia are called light rail lines. In North America, such systems are not usually considered light rail.

Many systems have mixed characteristics. Indeed, with proper engineering, a rail line could run along a street, then go underground, and then run along an elevated viaduct. For example, the Los Angeles Metro Rail's A Line "light rail" has sections that could alternatively be described as a tramway, a light metro, and, in a narrow sense, rapid transit. This is especially common in the United States, where there is not a popularly perceived distinction between these different types of urban rail systems. The development of technology for low floor and catenary free trams facilitates the construction of such mixed systems with only short and shallow underground sections below critical intersections as the required clearance height can be reduced significantly compared to conventional light rail vehicles. It is even possible to have high floor rapid transit cars run along a street, like a tram; this is known as street running.

Many light rail systems—even fairly old ones—have a combination of both on and off road sections. In some countries (especially in Europe), only the latter is described as light rail. In those places, trams running on mixed rights of way are not regarded as a light rail but considered distinctly as streetcars or trams. However, the requirement for saying that a rail line is "separated" can be quite low—sometimes just with concrete "buttons" to discourage automobile drivers from getting onto the tracks. Some systems such as Seattle's Link had on road mixed sections but were closed to regular road traffic, with light rail vehicles and traditional buses both operating along a common right of way (however, Link converted to full separation in 2019). Some systems, such as the AirTrain JFK in New York City, the DLR in London, and Kelana Jaya Line in Kuala Lumpur, have dispensed with the need for an operator. The Vancouver SkyTrain was an early adopter of driverless vehicles, while the Toronto Scarborough rapid transit operated the same trains as Vancouver, but used drivers. In most discussions and comparisons, these specialized systems are generally not considered light rail but as light metro systems.

Historically, the track gauge has had considerable variations, with narrow gauge common in many early systems. However, most light rail systems are now standard gauge.

One line of light rail (requires 7.6 m, 25' right of way) has a theoretical capacity of up to 8 times more than one 3.7 m (12 foot) lane on a freeway, excluding busses, during peak times. Roads have ultimate capacity limits that can be determined by traffic engineering, and usually experience a chaotic breakdown inflow and a dramatic drop in speed (a traffic jam) if they exceed about 2,000 vehicles per hour per lane (each car roughly two seconds behind another). Since most people who drive to work or on business trips do so alone, studies show that the average car occupancy on many roads carrying commuters is only about 1.5 people per car during the high demand rush hour periods of the day. This combination of factors limits roads carrying only automobile commuters to a maximum observed capacity of about 3,000 passengers per hour per lane. The problem can be mitigated by introducing high occupancy vehicle (HOV) lanes and ride sharing programs, but in most cases, policymakers have chosen to add more lanes to the roads, despite a small risk that in unfavorable situations an extension of the road network might lead to increased travel times (Downs–Thomson paradox, Braess paradox). By contrast, light rail vehicles can travel in multi car trains carrying a theoretical ridership up to 20,000 passengers per hour in much narrower rights of way, not much more than two car lanes wide for a double track system. They can often be run through existing city streets and parks, or placed in the medians of roads. If run in streets, trains are usually limited by city block lengths to about four 180 passenger vehicles (720 passengers). Operating on two minute headways using traffic signal progression, a well designed two track system can handle up to 30 trains per hour per track, achieving peak rates of over 20,000 passengers per hour in each direction. More advanced systems with separate rights of way using moving block signaling can exceed 25,000 passengers per hour per track.

An analysis of data from the 505 page National Transportation Statistics report published by the US Department of Transportation shows that light rail fatalities are higher than all other forms of transportation except motorcycle travel (31.5 fatalities per 100 million miles). However, the National Transportation Statistics report published by the US Department of Transportation states that: shows that costs of most LRT systems range from $15 million to over $100 million per mile. Seattle's new light rail system is by far the most expensive in the US, at $179 million per mile, since it includes extensive tunneling in poor soil conditions, elevated sections, and stations as deep as 180 feet (55 m) below ground level. This results in costs more typical of subways or rapid transit systems than light rail. At the other end of the scale, four systems (Baltimore, Maryland; Camden, New Jersey; Sacramento, California; and Salt Lake City, Utah) incurred construction costs of less than $20 million per mile. Over the US as a whole, excluding Seattle, new light rail construction costs average about $35 million per mile. However, freeways are frequently built in suburbs or rural areas, whereas light rail tends to be concentrated in urban areas, where right of way and property acquisition is expensive. Similarly, the most expensive US highway expansion project was the "Big Dig" in Boston, Massachusetts, which cost $200 million per lane mile for a total cost of $14.6 billion. A light rail track can carry up to 20,000 people per hour as compared with 2,000–2,200 vehicles per hour for one freeway lane. For example, in Boston and San Francisco, light rail lines carry 9,600 and 13,100 passengers per hour, respectively, in the peak direction during rush hour. The cost of 17 mile (27 km) of highway improvements and 19 mile (31 km) of double track light rail worked out to $19.3 million per highway lane mile and $27.6 million per LRT track mile. The project came in under budget and 22 months ahead of schedule. LRT cost efficiency improves dramatically as ridership increases, as can be seen from the numbers above: the same rail line, with similar capital and operating costs, is far more efficient if it is carrying 20,000 people per hour than if it is carrying 2,400. The Calgary, Alberta, C Train used many common light rail techniques to keep costs low, including minimizing underground and elevated trackage, sharing transit malls with buses, leasing rights of way from freight railroads, and combining LRT construction with freeway expansion. As a result, Calgary ranks toward the less expensive end of the scale with capital costs of around $24 million per mile. However, Calgary's LRT ridership is much higher than any comparable US light rail system, at 300,000 passengers per weekday, and as a result, its capital efficiency is also much higher. Its capital costs were one third those of the San Diego Trolley, a comparably sized US system built at the same time, while by 2009 its ridership was approximately three times as high. Thus, Calgary's capital cost per passenger was much lower than that of San Diego. Its operating cost per passenger was also much lower because of its higher ridership. A typical C Train vehicle costs only per hour to operate, and since it averages 600 passengers per operating hour, Calgary Transit estimates that its LRT operating costs are only 27 cents per ride, versus $1.50 per ride on its buses. and faster average speed (reducing the number of vehicles needed for the same service frequency). While light rail vehicles are more expensive to buy, they have a longer useful life than buses, sometimes making for lower life cycle costs. Compared to heavy rail investment costs are lower, however operating costs are higher than heavy rail.

Light rail lines have various policies on bicycles. Some fleets restrict bicycles on trains during peak hours. Some light rail systems, such as the St. Louis MetroLink, allow bicycles on the trains, but only in the rear sections of cars. Some light rail lines, like San Francisco's, allow only folding bicycles on board. Some light rail systems, such as the MAX in Portland, have dedicated bike storage, either with dedicated hooks or spaces within train cars. some systems, dedicated bike parking is available at select stations and others are integrated with local bike share systems.

When electric streetcars were introduced in the late 19th century, conduit current collection was one of the first ways of supplying power, but it proved to be much more expensive, complicated, and trouble prone than overhead wires. When electric street railways became ubiquitous, conduit power was used in those cities that did not permit overhead wires. In Europe, it was used in London, Paris, Berlin, Marseille, Budapest, and Prague. In the United States, it was used in parts of New York City and Washington, D. C. Third rail technology was investigated for use on the Gold Coast of Australia for the G:link light rail, though power from overhead lines was ultimately utilized for that system. In the French city of Bordeaux, the tramway network is powered by a third rail in the city center, where the tracks are not always segregated from pedestrians and cars. The third rail (actually two closely spaced rails) is placed in the middle of the track and divided into eight metre sections, each of which is powered only while it is completely covered by a tram. This minimizes the risk of a person or animal coming into contact with a live rail. In outer areas, the trams switch to conventional overhead wires. The Bordeaux power system costs about three times as much as a conventional overhead wire system and took 24 months to achieve acceptable levels of reliability, requiring the replacement of all the main cables and power supplies. Operating and maintenance costs of the innovative power system still remain high. However, despite numerous service outages, the system was a success with the public, gaining up to 190,000 passengers per day.

With its mix of right of way types and train control technologies, LRT offers the widest range of latitude of any rail system in the design, engineering, and operating practices. The challenge in designing light rail systems is to realize the potential of LRT to provide fast, comfortable service while avoiding the tendency to overdesign that results in excessive capital costs beyond what is necessary to meet the public's needs. Alternative Differences Rapid transit Light rail vehicles (LRVs) are distinguished from rapid rail transit (RRT) vehicles by their capability for operation in mixed traffic, generally resulting in a narrower car body and articulation to operate in a street traffic environment. With their large size, large turning radius, and often an electrified third rail, RRT vehicles cannot operate in the street. Since LRT systems can operate in existing streets, they can often avoid the cost of expensive grade separated subway and elevated segments that would be required with RRT. Streetcars or trams Conversely, LRVs generally outperform traditional streetcars in terms of capacity and top end speed, and almost all modern LRVs are capable of multiple unit operation. The latest generation of LRVs is considerably larger and faster, typically long with a maximum speed of around Heritage streetcarsA variation considered by many cities is to use historic or replica cars on their streetcar systems instead of modern LRVs. A heritage streetcar may not have the capacity and speed of an LRV, but it will add to the ambiance and historic character of its location. Light metroA derivative of LRT is light rail rapid transit (LRRT), also referred to as light metro. Such railways are characterized by exclusive rights of way, advanced train control systems, short headway capability, and floor level boarding. These systems approach the passenger capacity of full metro systems but can be cheaper to construct due to LRVs generally being smaller in size, turning tighter curves and climbing steeper grades than standard RRT vehicles, and having a smaller station size. InterurbansThe term interurban mainly refers to rail cars that run through streets like ordinary streetcars (trams), but also between cities or towns, often through rural environments. In the period 1900–1930, interurbans were very common in the US, especially in the Midwest. Some of them, like the Red Devils, the J. G. Brill Bullets, and the Electroliners, were the high speed railcars of their time, with an in service speed of up to about In Europe, interurbans are making a comeback as "tram trains" (locally known under different names) that operate on both the railway and light rail tracks, often with different voltages. The Karlsruhe Stadtbahn is one well known example.