The Altamont Corridor Express (also known as ACE, formerly Altamont Commuter Express) 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 a travel time of about 2 hours and 12 minutes end-to-end. In 2022, the line had a ridership of 389,100, or about 2,200 per weekday as of the second quarter of 2023. ACE uses Bombardier BiLevel Coaches, MPI F40PH-3C locomotives, and Siemens Charger locomotives.
Service 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. Saturday service commenced in September 2019. As of 2018, the average weekday ridership was 5,900.
The tracks are owned by Union Pacific, previously built along the 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.
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By the 1980s, three rapidly growing areas in California – Silicon Valley, the Tri-Valley, and the San Joaquin – were poorly connected by public transit, as Interstate 580 and Interstate 680 became more congested. Commuting from the San Joaquin Valley or the Tri-Valley to Silicon Valley required using a car or limited bus service.
In 1989, the San Joaquin Council of Governments, Stockton Chamber of Commerce, and the Building Industry Association of the Delta started work on a 20-year transportation plan for the northern section of the San Joaquin Valley. In November 1990, San Joaquin County voters passed Measure K, a half-cent sales tax to fund a variety of transportation improvements. The highest-priority project was the establishment of a passenger rail service to San Jose.
In 1995, San Joaquin County and seven cities along the route formed the San Joaquin Regional Rail Commission (SJRRC) to oversee the creation of the service. In May 1997, the Altamont Commuter Express Joint Powers Authority (ACE JPA) was formed by the SJRRC, Santa Clara Valley Transportation Authority (VTA), and Alameda Congestion Management Agency (ACMA). That agreement formalized financial support, administrative processes, and governance for the rail service. The operation is funded by a variety of state and federal sources, largely sales tax revenue collected by the three JPA signatories, while farebox revenues account for about one-third of costs.
Cost sharing for capital projects, excluding stations, during the initial 36 months of service was determined by the JPA on a case-by-case basis and approved by each of the member agencies. The initial purchase of rolling stock, construction of stations, and other start-up costs, amounting to some $48 million, were covered primarily by Measure K funds. Station improvements are the responsibility of the county in which the station is located. ACE pays the Union Pacific Railroad about $1.5 million per year to use their tracks. ACE trains also use about 4 miles (6.4 km) of Caltrain track in San Jose. Service began on October 19, 1998, with two daily round trips running to San Jose in the morning and Stockton in the evening.
