The San Diego Trolley is a light rail system operating in the metropolitan area of San Diego. It is known colloquially as The Trolley or by tourists as The Red Trolley. The 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. With an average of 89,400 riders per weekday as of Q2 2013, the trolley is the 7th most-ridden light rail system in the United States.