In rail transport, a medium-capacity system (MCS) is a non-universal term coined to differentiate an intermediate system between light rail and heavy rail. The concept is similar to Light Metro, seen in European countries (see section Variants of the term). A medium-capacity system is proposed when an area requires a rapid transit service but the predicted ridership falls between the gap of the other two rail tiers. In contrast with light rail, a medium-capacity system is usually running on an exclusive right-of-way. Furthermore, the distance between stations is much longer. An MCS may also be a branch connection to another mode of a heavy-capacity transportation system, such as an airport or the main route of a metro network.
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The definition of a medium-capacity system varies due to its non-standardization. This can exist even within a relatively small country. For example, the Taiwan Ministry of Transportation and Communications states that each train can board around 6,000 to 20,000 passengers per hour per direction (p/h/d).[1] However, the Taiwan Department of Rapid Transit Systems (TCG) suggests an MCS has a capability of boarding around 20,000 to 30,000 p/h/d.[2]
The train may have a shorter configuration than the standard metro system, usually 3 to 6 cars, allowing shorter platforms to be built. Rather than using steel wheels, rubber-tyred metro technology, such as the VAL system used on the Taipei Metro, is sometimes recommended, due to its low running noise, as well as the ability to climb steeper grades and turn tighter curves, thus allowing more flexible alignments.
The term may vary in different countries. In Russia, the "Light Metro" (Лёгкое метро) Л1 - Butovskaya Line has been built to serve the residents of outer Moscow. This line connects the passengers with the main routes of Moscow Metro. VAL, the French rubber-tyred fully automated metro system, also applies the term "Light Metro" to define its capacity (up to 30,000 p/h/d.[3]) These can thus also be categorized into the medium-capacity system family.
Light metros can generally operate faster than heavy-rail rapid transit systems, due to shorter dwell times at stations and the faster acceleration/deceleration of lighter trains. For example, express trains on the New York City Subway are about as fast as the Vancouver SkyTrain, but express trains skip most stops on lines where they operate.
Medium-capacity systems have a latent weakness in that as the service district's population increases, the increased transportation demand might create bottlenecks. But it is difficult to extend the platforms once in operation, since it must be done without interfering with traffic, especially for underground railway systems. Some railway planners may make provisions such as longer platforms than necessary so that they will be capable, in future, of accommodating trains with more cars or longer cars. The Taipei Rapid Transit System, for example, left extra space for two extra cars in all Wenshan Line and Neihu Line stations. The Ma On Shan Line in Hong Kong has even applied the railway standard (with less car configuration) for a possible link with the other existing heavy rail route without reconstructing the current system.
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