Heavy rail

Various terms are used for passenger rail lines and equipment-the usage of these terms differs substantially between areas:

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.

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.” 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.

...
Wikipedia