SAN FRANCISCO — The San Francisco Bay Area’s commuter train service was shut down by a strike on Monday, forcing hundreds of thousands of people onto overcrowded roads, bridges, buses and ferries. The dispute appeared headed into a second day as negotiations between the Bay Area Rapid Transit System and two of its largest unions broke down. No new talks were scheduled.

Some 2,400 employees of the commuter railroad, known as BART — which is the nation’s fifth-largest, used by more than 400,000 riders daily — went on strike after their four-year contract expired Sunday night. BART’s management and the two unions, representing train operators, station agents, maintenance and other workers, failed to reach an agreement on wages and benefits.

The labor organizations, the Service Employees International Union and the Amalgamated Transit Union, which were demanding a 5 percent raise in each of the next three years, rejected management’s offer of 2 percent annual increases over the next four years. Management is also pressing workers to start making contributions to their state pension plans and to increase their health insurance payments from the current level of $92 a month.