Light Rail Transport (LRT)
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LRT definition includes a wide range of applications, from classic tram to fast tram (metrotram). It appears correct to intend as properly said LRT systems those mass rapid transit systems which couldn't be classified as typical metro, not having a fully segregated track. They often present a combination of mixed traffic, reserved and segregated sections (e.g. short underground or viaduct segments); LRT systems are operated with tram o tram-like rolling stock.
The typical modern tram is a guided transport system which runs ground level (on reserved lanes or in mixed road traffic); in comparison with the traditional tramways, the modern ones assure higher commercial speed and higher capacity, thanks to an increased protection of its lanes (which can be partly reserved or segregated) in order to reduce its interferences with private cars and pedestrians. Each one of the major tram system producers tried to razionalize their market production by the development of the so-called "tram standard", a modular kind of vehicle easy to be adapted to the specific requirements of each purchaser. One of the distinguishing elements between the different models is the internal floor height referred to the rails level. Some of the modern trams (partial low floor) are more similar to the traditional ones, with the bogies able to rotate regards the car-body; in the traditional tramways, the floor height was up to 900 mm, in the modern partial low floor trams it's about 600 mm over the bogies, while it's only 350 mm in the rest of the vehicle (boarding zone included). A more innovative concept characterizes the full low floor trams, whose floor is about 350 mm height in every part of the car, due to the adoption of particolar bogies fixed to the carbody; the insertion of the vehicle in the curves is obtained thanks to suspended intermediate cars located between every two cars on bogies. Partial low floor trams generally maximize the cinematic performance and the cars durability, while full low floor trams assure a whole easy approachability in every part of the vehicle. Almost every modern tram model offers the double option of the mono and bi-directional type; the latter are more flexible (central and side platforms stops are both possible) but they must renounce at some seats in both side of the doors.
The so-called "tram on tyres" are a particular kind of LRTs/trams; they are also (and more correctly) called "intermediate systems" mixing trolleybus and tram features. The concept of tram on tyres is very recent and derives from the research of a flexible and reversible vehicle, which works mostly in guided mode (in order to maximize the system capacity, the performances and the minimization of the dynamic envelope) but able to switch in trolleybus mode, or, in some cases, running in the road traffic with autonomous power supply (batteries, diesel engine, etc.) like a normal bus. This feature could be very important in the ancient city centers of many towns, characterized by narrow roads and by pre-existing elements (e.g. buildings, monuments, etc.) which cause difficulties in the insertion of heavy infrastructures (such as the ordinary tram rails). Tram on tyres can be classified in order to their guidance system (mechanical guidance by a central monorail,optical guidance,magnetical guidance); at the present date, only the mechanical guidance systems seems to have a real diffusion, while the virtual guidance ones (optical or magnetical) encountered remarkable problems since their first applications.
The so-called "tram-trains" are another particular kind of LRTs/trams; their concept derives from the intention to integrate suburban railways (pre-existing lines which connect the urban centers with the districts areas) and tram lines (running inside the urban perimeter). This requires some fundamental common features between tram and local railway network, first of all the adoption of the same rail gauge. If the tram and the railway network have a different voltage, it's necessary the adoption of dual voltage vehicles, able to run both in tram mode (normally with 600/750 V DC overhead power supply) and in train mode (e.g. 15/25 KV AC if the line is electrified or with diesel autonomous power supply if the railway isn't electrified). Karlsruhe Stadbahn (Germany) is an emblematic case of properly said tram-train, with trams equipped with on-board inverters in order to switch from tram mode (750 V DC) to train mode (15 kV AC) and viceversa; this dual voltage cars share part of their route with suburban and regional ordinary trains. Kassel system (Germany) shows another interesting feature: the adopted rolling stock is able to run along the unelectrified railway sections switching from tram electric voltage (750 V DC) to an autonomous diesel power supply. In some other cases, the tram network simply reuses or incorporates some dismantled or under utilized railway tracks (e.g. Manchester Metrolink), without any real tram-train track sharing operations.