Palo Alto is the main train station in Palo Alto, California and the second busiest in the Caltrain system after the 4th and King Street Station in downtown San Francisco. It is a regional transit center where passengers can take buses serving Santa Clara County and San Mateo County as well as Caltrain commuters; in addition the Dumbarton Express bus administered by AC Transit takes passengers back and forth over the Dumbarton Bridge to Union City BART station in the East Bay.