Article: San Diego is served by the San Diego Trolley light rail system, by the SDMTS bus system, and by Coaster and Amtrak Pacific Surfliner commuter rail; northern San Diego county is also served by the Sprinter light rail line. The Trolley primarily serves downtown and surrounding urban communities, Mission Valley, east county, and coastal south bay. A planned Mid-Coast extension of the Trolley will operate from Old Town to University City and the University of California, San Diego along the I-5 Freeway, with planned operation by 2018. The Amtrak and Coaster trains currently run along the coastline and connect San Diego with Los Angeles, Orange County, Riverside, San Bernardino, and Ventura via Metrolink and the Pacific Surfliner. There are two Amtrak stations in San Diego, in Old Town and the Santa Fe Depot downtown. San Diego transit information about public transportation and commuting is available on the Web and by dialing "511" from any phone in the area.

Now answer this question: What areas of San Diego are primarily served by the trolley?
downtown and surrounding urban communities