Rail rapid transit is a term used to describe any form of mass transit that uses rail systems for transportation. There are several synonyms that can be used interchangeably with the term rail rapid transit, including light rail, tram, subway, metro, and rapid transit. Light rail refers to a system of urban transportation that typically involves smaller trains and serves communities with lower population densities. Tram is another option, often used in reference to streetcars that operate on rails. Subway and metro typically refer to systems of rapid transit that run beneath city streets and offer connections to other modes of transportation. Rapid transit is a more general term that encompasses any system of public transportation that operates quickly and efficiently.