The San Diego Trolley (reporting mark SDTI) is a light rail system operating in the metropolitan area of San Diego. It is known colloquially as The Trolley. The Trolley's operator, San Diego Trolley, Inc. (SDTI), is a subsidiary of the San Diego Metropolitan Transit System (MTS). The Trolley began service on July 26, 1981, making it the oldest of the "second generation" light rail systems in the United States. The entire Trolley network serves 53 stations, and comprises 53.5 miles (86.1 km) of route, and three primary lines named the Blue Line, the Orange Line, and the Green Line, as well as a supplementary heritage streetcar downtown circulator known as the Silver Line that operates on select weekdays, weekends and holidays. In Q4 2014, the Trolley was the 4th most-ridden light rail syste