Caltrain is a California commuter rail line serving the San Francisco Peninsula and Santa Clara Valley. The southern terminus is in San Jose at Tamien station with weekday rush hour service running as far as Gilroy. The northern terminus of the line is in San Francisco at 4th and King Street. Caltrain has 28 regular stops, one limited-service weekday-only stop, one weekend-only stop (Broadway), and one football-only stop (Stanford). While average weekday ridership in 2019 exceeded 63,000, impacts of the COVID-19 pandemic have been significant: in August 2022, Caltrain had an average weekday ridership of 18,600 passengers.

The San Francisco Peninsula is a peninsula in the San Francisco Bay Area that separates San Francisco Bay from the Pacific Ocean. On its northern tip is the City and County of San Francisco. Its southern base is Los Altos, Mountain View, in Santa Clara County, south of Palo Alto and north of Sunnyvale and Los Altos. Most of the Peninsula is occupied by San Mateo County, between San Francisco and Santa Clara counties, and including the cities and towns of Atherton, Belmont, Brisbane, Burlingame, Colma, Daly City, East Palo Alto, El Granada, Foster City, Hillsborough, Half Moon Bay, La Honda, Loma Mar, Los Altos, Menlo Park, Millbrae, Mountain View, Pacifica, Palo Alto, Pescadero, Portola Valley, Redwood City, San Bruno, San Carlos, San Mateo, South San Francisco, and Woodside.