The San Diego Trolley is the light rail system that serves the metropolitan area of San Diego. The operator of the Trolley, San Diego Trolley, Inc. (SDTI), is a subsidiary of the San Diego Metropolitan Transit System (MTS). The San Diego Trolley opened for service on July 26, 1981, Today operates three main 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 mid-days on Tuesdays & Thursdays, and on weekends and holidays.