A rapid transit system is an electric railway characterized by high speed and rapid acceleration. It uses passenger railcars operating singly or in multiple unit trains on fixed rails. It operates on separate rights-of-way from which all other vehicular and foot traffic are excluded (i.e. is fully grade separated from other traffic). It uses sophisticated signaling systems, and high platform loading.

Originally, the term rapid transit was used in the 1800s to describe new forms of quick urban public transportation that had a right-of-way separated from street traffic. This set rapid transit apart from horsecars, trams, streetcars, omnibuses, and other forms of public transport.

Though the term was almost always used to describe rail transportation, other forms of transit were sometimes described by their proponents as rapid transit, including local ferries in some cases.

The term bus rapid transit has recently come into use to describe bus lines with features to speed their operation. These usually have more characteristics of light rail than rapid transit.

Metro

Metros, short for metropolitan railways, are defined by the International Association of Public Transport (L'Union Internationale des Transports Publics, or UITP) as urban guided transport systems "operated on their own right of way and segregated from general road and pedestrian traffic. They are consequently designed for operations in tunnel, viaducts or on surface level but with physical separation in such a way that inadvertent access is not possible. In different parts of the world Metro systems are also known as the underground, the subway or the tube. Rail systems with specific construction issues operating on a segregated guideway (e.g. monorail, rack railways) are also treated as Metros as long as they are designated as part of the urban public transport network."[2] Metropolitan railways are used for high capacity public transportation. They can operate in trains of up to 10 cars, carrying 1800 passengers or more. In Germany, the terms U-Bahn and S-Bahn are used. Some metro systems run on rubber tires but are based on the same fixed-guideway principles as steel wheel systems.

Subway

Subway used in a transit sense refers to either a rapid transit system using heavy rail or a light rail/streetcar system that goes underground. The term may refer only to the underground parts of the system, or to the full system. Subway is most commonly used in the United States and the English-speaking parts of Canada, though the term is also used elsewhere, such as to describe the SPT Subway in Glasgow, Scotland, and in translation of system names or descriptions in some Asian and Latin American cities.

Some lines described as subway use light rail equipment. Notably, Boston's Green Line and the Newark City Subway, each about half underground, originated from fully surface streetcar lines. Also, the Buffalo Metro Rail is referred to as "the subway", while it uses light rail equipment and operates in a pedestrian mall downtown for half of its route and underground for the remaining section. Sometimes the term is qualified, such as in Philadelphia, where trolleys operate in an actual subway for part of their route and on city streets for the remainder. This is locally styled subway-surface.

In some cities where subway is used, it refers to the entire system; in others, only to the portions that actually are underground. Naming practices often select one type of placement in a system where several are used; there are many subways with above-ground components, and on the other hand, the Vancouver SkyTrain and Chicago 'L' include underground sections. Historic posters referred to Chicago's Red & Blue lines (then called the State Street &Milwaukee/Dearborn lines) as "the subway lines".

Interestingly, when the Boston subway was originally built, the subway label was only used for sections into which streetcars (trams) operated, and the rapid transit sections were called tunnels. Also, in some countries, subway refers to systems built under roads and the informal term tube is used for the deep-underground tunnelled systems (such as London's Piccadilly line) – in this usage, somewhat technical nowadays and not used much in London, underground is regardless the general term for both types of system.

Bus subways are uncommon but do exist, though in these cases the non-underground portions of route are not called subways. Seattle, Washington, has a bus subway downtown, in which light rail trains and diesel-electric hybrid buses operate in a shared tunnel, with overhead wires which power the light rail trains, and the hybrid buses running in electrical-only mode while traveling through the tunnel. Bus subways are sometimes built to provide an exclusive right-of-way for bus rapid transit lines, such as the MBTA Silver Line in Boston. These are usually called by the term bus rapid transit.

'Subway' outside the USA, and especially in Europe often refers to an underground pedestrian passageway linking large road interconnections that are often too difficult or dangerous to cross at ground level. In Canada, the term subway may be used in either sense.

Underground and Tube

The usage of underground is very similar to that of subway, describing an underground train system.

In London the colloquial term tube now refers to the London Underground and is the most common word used for the underground system, and it is used by Transport for London the local government body responsible for most aspects of the transport system throughout Greater London.[3] However, strictly speaking, it should only refer to those deep lines which run in bored circular tunnels as opposed to those constructed near to the surface by 'cut-and-cover' methods.[4] The Glasgow metro system is known as the Glasgow Subway or colloquial as "the subway". The word Metro is not usually used in London or Glasgow to refer to those cities' metros, but it is used in and around Newcastle upon Tyne to refer to the Tyne and Wear Metro.

Paris, Rome, Madrid, Barcelona, Copenhagen, Helsinki, Warsaw, Saint Petersburg, Rotterdam and Moscow all have metro (from the word metropolitan) systems which are called metro in French, Italian, Spanish, Danish, Finnish, Polish, Dutch and Russian.

U-Bahn and S-Bahn

The term metro is not usually used to describe metro systems in German-speaking areas (Germany, Austria and parts of Switzerland), instead using the term U-Bahn—a shortening of Untergrundbahn, meaning "underground railway"—and S-Bahn—an abbreviation for the German "Stadtschnellbahn" or just "Schnellbahn" (fast city train, fast train) the more common English translation, suburban train. So for example in Berlin, the mostly underground system is known as the Berlin U-Bahn and it is integrated with the mostly above-ground system, known as the Berlin S-Bahn.

Hamburg S-Bahn fulfills all criteria for heavy rail inside the state and city of Hamburg, but some lines go beyond the state border into the state of Niedersachsen and there the S-Bahn runs with lower train frequency.

The same applies also to the S-Bahn and U-Bahn in Copenhagen, Denmark, with the only exception that the word "Metro" is used instead of "U-Bahn" and "S-tog" instead of "S-Bahn". (The Danish word "S-tog" applies to the trains (tog), rather than the tracks as in Germany; "S-tog" means "S-train".) Otherwise, the S-Bahn of Berlin and the S-tog of Copenhagen are very similar with the exception of the size.

In Switzerland, where there is only one underground railway system in Lausanne, the term metro is generally used, due to the influence from the French language.

In Sweden, the metro of Stockholm is called "Tunnelbana" or "T-bana" which applies to the fact that the trains often runs in tunnels. The same applies to Norway and the "T-bane" of Oslo.

Elevated and Overhead

Elevated is a shorthand for elevated railway, a railway built on supports over other rights of way, generally city streets. The term overhead tends to be used in Europe. The names of elevated railways are sometimes further abbreviate it to El or L. Some examples include:

Chicago 'L' The best known elevated transit system in the United States.

Vancouver SkyTrain An automated rapid transit system that is mostly elevated.

New York City Subway A combination of the old IRT and BMT rapid transit systems that had built or leased numerous elevated lines throughout the entire city. New York "El's" are the oldest ones in the United States, dating from 1869. Today, the majority of "El" lines in New York are in Brooklyn, Queens, and The Bronx. Most "El's" in Manhattan were torn down in the 1940s and '50s, some replaced by subways.

Liverpool Overhead Railway This was the United Kingdom's only true elevated railway, although the London and Greenwich Railway of 1836 was constructed on a 3.45 miles (5.55 km) brick viaduct for the greater part of its length.

SEPTA's Market-Frankford Line is elevated except for the portion running through Center City and University City, and is sometimes referred to as the "El".

The Manila LRT Line 1 in Manila, Philippines, is an elevated railway, made operational in 1984 and the country's first urban rail transit since Manila tram service ended in 1944, during the Japanese occupation of the city.

The BTS Skytrain is an elevated rapid transit system in Bangkok, Thailand, which was officially opened on 5 December 1999 by Princess Maha Chakri Sirindhorn. It now consists of 34 stations and 2 lines.