Altamont Corridor Express

Altamont Corridor Express
ACE train climbing its namesake Altamont Pass in 2010
Overview
Owner	San Joaquin Regional Rail Commission
Area served	San Joaquin Valley, Tri-Valley and Silicon Valley
Transit type	Commuter rail
Number of stations	10
Daily ridership	2,900 (weekdays, Q2 2026)
Annual ridership	763,800 (2025)
Headquarters	Robert J. Cabral Station in Stockton, California
Website	acerail.com
Operation
Began operation	October 19, 1998 (1998-10-19)
Operator	Herzog Transit Services
Reporting marks	ACEX
Infrastructure managers	Union Pacific (Stockton–Santa Clara) Caltrain (Santa Clara–San Jose)
Number of vehicles	10 locomotives, 30 passenger cars
Train length	1–2 locomotives, 4–8 passenger cars
Technical
System length	85 mi (137 km)
Track gauge	4 ft 8+1⁄2 in (1,435 mm) standard gauge
Average speed	39 mph (63 km/h)
Top speed	79 mph (127 km/h)
System map Altamont Corridor Express highlighted in purple Valley Rail (2026) Amtrak Coast Starlight to Seattle Bus connection to Sacramento Airport Natomas/Sacramento Airport Old North Sacramento (2029) Globe (RT) RT to Watt/I-80 Amtrak to Auburn & Chicago American River to Downtown Sacramento Midtown Sacramento () SacRT to Historic Folsom City College (2029) SacRT to Cosumnes River College Elk Grove Sacramento County San Joaquin County Lodi (2027) ACE service facility Amtrak Gold Runner to Sacramento Stockton Amtrak Gold Runner Oakland │ Bakersfield Valley Rail (2026/2030) Valley Rail (2026/2030) North Lathrop (2027) Manteca Transit Center Ripon (2027) Stanislaus R San Joaquin Co. Stanislaus Co. Modesto Tuolumne River Ceres layover facility bus bridge Phase 1 (2026)   Turlock (2029)   Phase 2 (2030) Stanislaus County Merced County Merced River Livingston Atwater (option) Layover and maintenance facility Merced CAHSR Lathrop/Manteca San Joaquin River Tracy California Aqueduct San Joaquin County Alameda County Altamont Pass through the Diablo Range Vasco Road Livermore Pleasanton Niles Canyon Amtrak to Oakland & Seattle Union City (planned) Fremont Amtrak (planned) Coyote Creek Alameda County Santa Clara County VTA to Mountain View Lick Mill (VTA) Great America Caltrain to San Francisco Santa Clara VTA to Santa Teresa San Jose VTA to Winchester Caltrain to Gilroy Coast Starlight to Los Angeles All stations are accessible

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.