It can be hard to distinguish light rail from other forms of urban and commuter rail. A system termed light rail in one city may be considered a streetcar or tram system in another. Conversely, some lines that are called light rail are very close to
rapid transit. In recent years, new terms such as
light metro have been used to describe medium-capacity systems. Some diesel light rail systems, such as
Sprinter, bear little similarity to urban rail, and could alternatively be classified as commuter rail with lightweight trains. There is a history of what would now be considered light rail vehicles operating on heavy rail
rapid transit tracks in the US, especially in the case of
interurban streetcars. Notable examples are
Lehigh Valley Transit trains running on the
Philadelphia and Western Railroad high-speed third rail line (now the
Norristown High-Speed Line). operates over in Belgium connecting several town centres
Flexibility Light rail corridors may constitute a fully segregated corridor, a dedicated
right-of-way on a street, an on-street corridor shared with other traffic, a corridor shared with other public transport, or a corridor shared with pedestrians, resulting in a much higher flexibility than
heavy 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 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.
Lower capacity The most difficult distinction to draw is that between low-floor light rail and streetcar or tram systems. There is a significant amount of overlap between the technologies; similar rolling stock may 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. However, some distinctions can be made, though systems may combine elements of both. Low-floor light rail lines tend to follow a reserved right-of-way and with trains receiving priority at intersections, and tend not to operate in mixed traffic, enabling higher operating speeds. Light rail lines tend to have less frequent stops than tramways, and operate over a longer distance. Light rail cars are often coupled into multiple units of two to four cars. 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.
Light rail operating on mainline railroads , trams can share mainline tracks with heavy rail trains. in
Sydney runs mostly along a former heavy rail corridor. Light rail can be designed to address a gap in interurban transportation between mainline rail and bus services, carrying high passenger numbers more quickly than local buses and more cheaply than mainline trains. Around
Karlsruhe,
Kassel, and
Saarbrücken in Germany, dual-voltage light rail trains partly use mainline railroad tracks, sharing these tracks with mainline rail trains. This allows commuters to ride directly into the city center, rather than taking a mainline train only as far as a central station and then having to change to a tram. In France, similar
tram-trains are in use in Paris,
Mulhouse, and
Strasbourg; further plans exist. In 2022, Spain opened the
Cádiz TramBahia, where trams share track with
commuter and long-distance trains from the main terminus in the city and curve off to serve cities without a rail connection. Some of the issues involved in such schemes are: • compatibility of the safety systems • power supply of the track to the power used by the vehicles (frequently different voltages, rarely third rail vs overhead wires) • width of the vehicles to the position of the
platforms • height of the platforms In some cases, tram-trains use previously abandoned or lightly used heavy rail lines in addition to or instead of active mainline tracks, like the
Manchester Metrolink. In the
San Diego region, the
San Diego and Arizona Eastern Railway was used to create the initial stage of the light rail network of the
San Diego Trolley. In the
Netherlands, this concept was first applied on the
RijnGouweLijn, and was followed by the
RandstadRail project.
System-wide considerations 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, whereas trams running on mixed rights-of-way are not. However, the requirement for saying that a rail line is "separated" can be quite low—sometimes concrete "buttons" to discourage automobile drivers from getting onto the tracks will suffice. Some systems, such as
Seattle's Link, had on-road mixed sections but were closed to regular road traffic, with light rail vehicles and buses both operating along a common right-of-way. The Link was converted to full separation in 2019.
Floor height system. Low-floor LRVs have the advantage of a low-floor design, allowing them to load passengers directly from low-rise platforms that can be little more than raised curbs. High-floor light rail systems also exist, featuring larger stations. However, low top speed is not always a differentiating characteristic between light rail and other systems. For example, the
Siemens S70 LRVs used in the
Houston METRORail and other North American LRT systems have a top speed of depending on the system, while the trains on the all-underground
Montreal Metro can only reach a top speed of .
LACMTA light rail vehicles have higher top and average speeds than Montreal Metro or
New York City Subway trains. == Infrastructure ==