Altamont Corridor Express

The Altamont Corridor Express (ACE) is a commuter rail service in California, connecting Stockton and San Jose during peak hours only. ACE is named for the Altamont Pass, through which it runs. Service is managed by the San Joaquin Regional Rail Commission, and operations are contracted to Herzog Transit Services. The 86-mile (138 km) route includes ten stops, with travel time about 2 hours and 12 minutes end-to-end. In 2025, the line had a ridership of 763,800, or about 2,900 per weekday as of the second quarter of 2026. ACE uses Bombardier BiLevel Coaches, MPI F40PH-3C locomotives, and Siemens Charger locomotives. Altamont Commuter Express began on October 19, 1998, with two weekday round trips. A third round trip was added in May 2001, followed by a fourth round trip in October 2012. The service was rebranded as Altamont Corridor Express in 2012. Saturday service began in September 2019, but was suspended in March 2020 due to the outbreak of COVID-19. The tracks are owned by Union Pacific Railroad, previously built along the Western Pacific Railroad main line. Under the ACEforward program, a number of improvements to the service are being considered. These include a rerouted line through Tracy, an extension to Modesto and Merced, and connections to BART at Union City and Tri-Valley.