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SAN DIEGO — Sunday brings the much-anticipated debut of the UC San Diego Blue Line extension, with nine new trolley stops from the U.S.-Mexico border up to University City.

Riders can take the trolley for free all day on any line on opening day of the new extension, Nov. 21.

SANDAG and the Metropolitan Transit System are hosting a grand opening celebration with free food, live music, merchandise giveaways, activities for kids — including face painting and a rock climbing wall — and addresses from public officials Sunday. It will run from 11 a.m. to 3 p.m. at the new UC San Diego Central Campus location. Attendees can ride to that station for free and then follow signs to the celebration.

The event isn’t ticketed, but MTS is asking people to RSVP online so they have an accurate idea of expected turnout. Face masks are required on the trolley and recommended at the grand opening event, MTS said. Residents were asked to leave pets at home.

Sunday’s free ride offer does not apply to MTS bus trips. An updated list of service changes is available online.

Built by SANDAG for trolleys operated by MTS, the 11-mile extension took over five years and cost more than $2 billion to complete. It extends the Blue Line from the border through downtown and along the busy Mid-Coast corridor, with new stations at Tecolote Road, Clairemont Drive, Balboa Avenue, Nobel Drive, VA Medical Center, UC San Diego Central Campus, UC San Diego Health La Jolla, Executive Drive and the UTC transit center.

You can view a map of the extended Blue Line and check trolley schedules on the MTS website.