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),[1] is a subsidiary of the San Diego Metropolitan Transit System (MTS). The San Diego Trolley opened for service on July 26, 1981,[1] Today 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]