Light rail in the United States is a mode of rail-based transport, usually urban in nature. When compared to heavy rail systems like commuter rail or rapid transit (subway), light rail systems are typically are designed to carry fewer passengers and are capable of operating in mixed traffic (street running) or on routes that are not entirely grade-separated. Systems typically take one of four forms: the "first-generation" legacy systems, the "second-generation" modern light rail systems, streetcars, and hybrid rail systems (light rail with some commuter rail features). All of the systems use similar technologies, and some systems blur the lines between the different forms.