The San Diego Trolley is a trolley-style light rail system operating in the metropolitan area of San Diego, California. The operator, San Diego Trolley, Inc. (SDTI) is a subsidiary of the San Diego Metropolitan Transit System (MTS). The Trolley began revenue service on July 26, 1981, making it the first modern light-rail system in California. It is the fifth most-ridden light rail system in the country.