Tram lines may also run between cities and/or towns (for example, interurbans, tram-train), and/or partially grade-separated even in the cities (light rail). Very occasionally, trams also carry freight. Tram vehicles are usually lighter and shorter than conventional trains and rapid transit trains, but the size of trams (particularly light rail vehicles) is rapidly increasing. Some trams (for instance tram-trains) may also run on ordinary railway tracks, a tramway may be upgraded to a light rail or a rapid transit line, two urban tramways may be connected to an interurban, etc. For all these reasons, the differences between the various modes of rail transportation are often indistinct. In the United States, the term tram has sometimes been used for rubber-tired trackless trains, which are not related to the other vehicles covered in this article.
Today, most trams use electrical power, usually fed by an overhead pantograph; in some cases by a sliding shoe on a third rail, trolley pole or bow collector. If necessary, they may have dual power systems — electricity in city streets, and diesel in more rural environments. Trams are now included in the wider term "light rail",[full citation needed] which also includes segregated systems.
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