The San Diego Trolley is a light rail system operating in the metropolitan area of San Diego. The trolley's operator, San Diego Trolley, Inc. (reporting mark SDTI), is a subsidiary of the San Diego Metropolitan Transit System (MTS). The San Diego Trolley opened for service on July 26, 1981.[1] Today, it operates three main lines named the Blue, Green, and Orange lines, as well as a supplementary heritage streetcar downtown circulator known as the Silver Line that operates on holidays.[1]