The San Diego Trolley is a light rail system operating in San Diego County, California. 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] It operates four primary lines (Blue, Green, Orange, and Copper) as well as a supplementary heritage streetcar downtown loop known as the Silver Line that operates on holidays.[1]