Light metro

Since ridership determines the scale of a rapid transit system, statistical modeling allows planners to size the rail system for the needs of the area. When the predicted ridership falls between the service requirements of a light rail system and a rapid transit system, a light metro project is indicated. A light metro system may also result when a rapid transit system fails to achieve the requisite ridership due to network inadequacies (e.g. single-tracking) or changing demographics.

In contrast with light rail systems, a light metro system runs on an entirely grade separated exclusive right of way, and is therefore completely separated from other traffic. In some cases, the distance between stations is much longer than typically found on rapid transit systems. A light metro system may also be suitable for branch line connections to another mode of a heavy-capacity transport system, such as an airport or a main route of a metro network. However, despite their smaller trains, some light metro systems can rival rapid transit networks in terms of capacity thanks to automatic train operation allowing extremely high-frequency service.

Definition

Light metro systems typically feature shorter trains and smaller stations than rapid transit and thus, lower capacity; this is typically their defining feature, especially compared to other rail transport in the same area. Automatic train operation is more common on light metro systems than rapid transit systems, which has allowed some systems to achieve extremely high frequency, with headways as close as 90 seconds during peak hours, making them competitive with traditional human-driven metro systems in terms of capacity.

Rubber-tyred metro technology, such as the VAL system used on the Taipei Metro, is sometimes used for light metro systems, due to its low running noise, as well as the ability to climb steeper grades and turn tighter curves, thus allowing more flexible alignments.

Rapid transit systems generally have train headways of 10 minutes or better during peak hours. Some systems that qualify as rapid transit in other technical respects (e.g. are fully grade separated), but which have network inadequacies (e.g. a section of single track rail) which cap headways, resulting in decreased capacity, and thus would be more accurately defined as light metro systems as a result.

Capacity

A report from the World Bank places the capacity of a light metro system at 15,000 to 30,000 p/h/d (passengers per hour per direction or PPHPD). For comparison, ridership capacity of more than 30,000 p/h/d has been quoted as the standard for rapid transit systems, while light rail systems have passenger capacity volumes of around 10,000 to 12,000 p/h/d or 12,000 to 18,000 p/h/d. VAL (Véhicule Automatique Léger) systems are categorised as light metro because their manufacturer defines their passenger capacities as being up to 30,000 p/h/d.

However, the capacity boundaries for a line to be categorised as a light metro system varies according to the standard used, sometimes even within a single country. For example, the Taiwan Ministry of Transportation and Communications states that each "medium-capacity system" can board around 6,000 to 20,000 p/h/d, while the Taiwan Department of Rapid Transit Systems defines a capacity of 20,000 to 30,000 p/h/d, which approaches many rapid transit systems, as "medium-capacity systems".

In Hong Kong, MTR's Ma On Shan line was locally classified as a "medium-capacity system" (as it used shorter 4-car SP1950 trains, compared to 7- to 12-car trains on other MTR lines) but can attain up to 32,000 p/h/d which is comparable to the passenger capacity of some rapid transit systems. However, it was built to the full rapid transit standard as it was designed to be extended. Full-length, 8-car trains were deployed on the line in advance of its extension into the Tuen Ma line in June 2021. Two other lines, the Disneyland Resort line shuttle service since 2005 and the South Island line since December 2016, are also classified as "medium-capacity systems" because of their shorter trains and smaller capacity, however they use the same technology as the other rapid transit lines.