The Altamont Corridor Express (ACE) is a
commuter rail
Commuter rail or suburban rail is a Passenger train, passenger rail service that primarily operates within a metropolitan area, connecting Commuting, commuters to a Central business district, central city from adjacent suburbs or commuter town ...
service in
California
California () is a U.S. state, state in the Western United States that lies on the West Coast of the United States, Pacific Coast. It borders Oregon to the north, Nevada and Arizona to the east, and shares Mexico–United States border, an ...
, connecting
Stockton and
San Jose during peak hours only. ACE is named for the
Altamont Pass
Altamont Pass, formerly Livermore Pass, is a low mountain pass in the Diablo Range of Northern California between Livermore in the Livermore Valley and Tracy in the San Joaquin Valley. The name is actually applied to two distinct but nea ...
, through which it runs. Service is managed by the San Joaquin Regional Rail Commission, and operations are contracted to Herzog Transit Services.
The route includes ten stops, with travel time about 2 hours and 12 minutes end-to-end. In , the line had a ridership of , or about per weekday as of . 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
The Union Pacific Railroad is a Railroad classes, Class I freight-hauling railroad that operates 8,300 locomotives over routes in 23 U.S. states west of Chicago and New Orleans. Union Pacific is the second largest railroad in the United Stat ...
, previously built along the
Western Pacific Railroad
The Western Pacific Railroad was a Class I railroad in the United States. It was formed in 1903 as an attempt to break the near-monopoly the Southern Pacific Railroad had on rail service into northern California. WP's Feather River Route dire ...
main line. Under the ACE''forward'' program, a number of improvements to the service are being considered. These include a rerouted line through Tracy, an extension to
Modesto
Modesto ( ; ) is the county seat and largest city of Stanislaus County, California, United States. With a population of 218,069 according to 2022 U.S. Census Bureau estimates, it is the 19th-most populous city in California.
Modesto is locate ...
and
Merced
Merced (; Spanish language, Spanish for "Mercy") is a city in, and the county seat of, Merced County, California, United States, in the San Joaquin Valley. As of the 2020 United States census, 2020 census, the city had a population of 86,333, up ...
, and connections to BART at Union City and Tri-Valley.
History and funding
Planning
By the 1980s, three rapidly growing areas in California –
Silicon Valley
Silicon Valley is a region in Northern California that is a global center for high technology and innovation. Located in the southern part of the San Francisco Bay Area, it corresponds roughly to the geographical area of the Santa Clara Valley ...
, the
Tri-Valley, and the
San Joaquin Valley
The San Joaquin Valley ( ; Spanish language in California, Spanish: ''Valle de San Joaquín'') is the southern half of California's Central Valley (California), Central Valley. Famed as a major breadbasket, the San Joaquin Valley is an importa ...
– 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
San Joaquin County ( ; , meaning " St. Joachim"), officially the County of San Joaquin, is a county located in the U.S. state of California. As of the 2020 United States census, the population was 779,233. The county seat is Stockton.
San Jo ...
voters passed Measure K, a half-cent sales tax to fund a variety of transportation improvements.
The highest-priority project was the establishment of 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
The Santa Clara Valley Transportation Authority, more commonly known simply as the Valley Transportation Authority (VTA), is a Special district (United States), special district responsible for public transit services, Congestion management agen ...
(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 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.
The service was named Altamont Commuter Express after the
Altamont Pass
Altamont Pass, formerly Livermore Pass, is a low mountain pass in the Diablo Range of Northern California between Livermore in the Livermore Valley and Tracy in the San Joaquin Valley. The name is actually applied to two distinct but nea ...
through which it runs.
Service expansion

The original service used two trainsets, each with four bilevel coach cars, for a total seated capacity of 1,120 passengers in each direction daily. In September 1999, less than a year after opening, the service reached 1,000 daily riders per direction, near full capacity.
To enable more trains on the line, ACE funded $3 million in track improvements, but the limited amount of equipment allowed ACE to add only a morning "
short turn" run between San Jose and Pleasanton. This "turn-back train" started service on February 21, 2000, and gave Pleasanton and Fremont a third inbound train to alleviate the crowding on the two earlier trains. Even with the added capacity, by early 2001 ACE was regularly carrying more than 700 daily standees.
ACE purchased additional equipment, allowing the "turn-back train" to operate to Lathrop/Manteca station – nearly the full length of the route – beginning on March 5, 2001. Trains also began stopping at
Santa Clara station. Although the third train added 560 seats in each direction, it brought an immediate increase of 380 daily riders. ACE then planned to add a fourth round trip later in the year, with fifth and sixth round trips by 2006.
However, by late 2001, the deepening
dot-com recession was hurting ridership, and expansion plans were put on hold. On June 30, 2003, the ACE JPA was dissolved in favor of a Cooperative Services Agreement between the three member agencies.
On January 6, 2003, ACE introduced the Stockton Solution Shuttle, allowing Stockton passengers to use the ACE trip which terminated at Lathrop/Manteca.
The rail trip was extended to Stockton on August 1, 2005. At that time, service to Santa Clara was suspended to allow for the construction of a second platform and pedestrian tunnel at the station.
On August 28, 2006, ACE added a fourth round trip, which operated midday using one of the existing trainsets.
On November 7, 2006, San Joaquin County voters approved a 20-year extension of Measure K.
Suffering from reducing funding due to the
Great Recession
The Great Recession was a period of market decline in economies around the world that occurred from late 2007 to mid-2009. , ACE cut the lightly used midday trip on November 2, 2009.
On May 14, 2012, ACE restored service to Santa Clara station.
On October 1, 2012, a fourth rush-hour round trip was added, running approximately one hour after existing trips.
}
Altamont Corridor Express
In December 2012, the service was rebranded from Altamont Commuter Express to Altamont Corridor Express to reflect plans for a broader scope of service.
In March 2014, ACE opened a $65 million, maintenance facility in Stockton. On July 1, 2015, management and governance of the ''San Joaquin'' passed from Caltrans to the new San Joaquin Joint Powers Authority. The SJRRC continued to handle normal operation and administration. On March 7, 2016, an ACE train was derailed by a mudslide in
Niles Canyon near
Sunol. The front car plunged into the rain-swollen
Alameda Creek
Alameda Creek () is a large perennial stream in the San Francisco Bay Area. The creek runs for from a lake northeast of Packard Ridge to the eastern shore of San Francisco Bay by way of Niles Canyon and a flood control channel. Along its course, ...
. Fourteen passengers were injured, but there were no fatalities.
ACE received
Road Repair and Accountability Act
A road is a thoroughfare used primarily for movement of traffic. Roads differ from streets, whose primary use is local access. They also differ from stroads, which combine the features of streets and roads. Most modern roads are paved.
The w ...
funds in January 2018 to begin Saturday service. Two Saturday round trips were added on September 7, 2019. Saturday service was suspended effective March 21, 2020, due to the COVID-19 pandemic. One weekday round trip was suspended on March 23 and another on April 6. One of the suspended weekday round trips returned on May 3, 2021, followed by the fourth round trip on September 7, 2021. On November 18, 2024, the latest evening eastbound train was replaced with a mid-afternoon train.
Future plans
ACEforward
In association with the
California High-Speed Rail
California High-Speed Rail (CAHSR) is a publicly funded high-speed rail system being developed in California by the California High-Speed Rail Authority. Phase 1, about long, is planned to run from San Francisco, California, San Francisco to ...
project, ACE developed plans to upgrade and expand service. Beginning around 2008, initial plans called for the Altamont Corridor Rail Project to produce a
high speed rail "Super ACE" capable of halving the travel time between the endpoints.
As the high-speed rail project was scaled back and rerouted to
Pacheco Pass several years later, these plans were replaced with the more modest ACE''forward'' program. The San Joaquin Regional Rail Commission issued a notice of intent to proceed with an
Environmental Impact Statement
An environmental impact statement (EIS), under United States environmental law, is a document required by the 1969 National Environmental Policy Act (NEPA) for certain actions "significantly affecting the quality of the human environment". An E ...
in June 2013; this was released in 2017 and prioritized goals as either long term or short term.
Short term goals included track improvements, a possible reroute through downtown
Tracy including new stations, a West Tracy station, and a new extension to
Modesto
Modesto ( ; ) is the county seat and largest city of Stanislaus County, California, United States. With a population of 218,069 according to 2022 U.S. Census Bureau estimates, it is the 19th-most populous city in California.
Modesto is locate ...
in addition to additional daily round trips. Long term goals included upgrades to the existing corridor to allow as many as 10 daily round trips, extension to
Merced
Merced (; Spanish language, Spanish for "Mercy") is a city in, and the county seat of, Merced County, California, United States, in the San Joaquin Valley. As of the 2020 United States census, 2020 census, the city had a population of 86,333, up ...
and
electrification
Electrification is the process of powering by electricity and, in many contexts, the introduction of such power by changing over from an earlier power source. In the context of history of technology and economic development, electrification refe ...
of the line from Stockton to San Jose.
Also under studied were possible connections with
BART
Bay Area Rapid Transit (BART) is a rapid transit system serving the San Francisco Bay Area in California. BART serves List of Bay Area Rapid Transit stations, 50 stations along six routes and of track, including eBART, a spur line running t ...
at
Union City or the
Tri-Valley via traditional ACE rail,
diesel multiple units, or bus bridges. This connection is planned to be facilitated by the Tri-Valley–San Joaquin Valley Regional Rail Authority.
The California state senate allocated $400 million in revenue from a
gas tax increase to ACE''forward'' expansion.
By 2019, the plan had come to be called the Altamont Corridor Vision, with an expected price of $9.7 billion, allowing ACE to run up to six weekday round trips in 2023 with the goal of ten weekday round trips once additional track infrastructure is completed. ACE and the Tri-Valley–San Joaquin Valley Regional Rail Authority sought funding to construct a shared tunnel under Altamont pass in order to speed service and increase reliability.
Valley Rail
However, during the development of ACE''forward'', significant financial and logistical challenges to expanding service on the existing route between Stockton and San Jose were identified, and further work on the project was halted in favor of a new
Valley Rail project, focusing initially on the eastern expansion to serve commuters living in the
Central Valley.
ACE was awarded $500.5 million in April 2018 for expanded service to Ceres and Sacramento to provide more rail service and connections within the
Central Valley. Service is expected to begin from Ceres by 2023
with interim
bus bridge service to Merced until that segment of Union Pacific right-of-way is upgraded. Four trains will depart Ceres in the mornings, and one train may make the complete run to San Jose with others transferring passengers at North Lathrop.
The Union Pacific right of way between Ceres and Lathrop will be
double tracked to facilitate passenger service.
platforms are being extended to accommodate longer trains.
Valley Rail also includes a project segment to route ACE and Amtrak ''
San Joaquins'' along the little-used
Sacramento Subdivision between Stockton and Sacramento. Six new stations would be constructed along the line with a layover facility at Natomas. Trains would run the length of the line from Natomas to San Jose or Ceres with a midday
short turn to Stockton. A Draft Environmental Impact Statement was released in 2020, with services expected to begin no later than 2023. The North Elk Grove station was eliminated from planning in September 2020. The project received funding via California's Transit and Intercity Rail Capital Program in 2023, by which time the project was expected to open in phases beginning in 2025. Later that year, the estimated commencement of service to Ceres and Natomas were again pushed back to 2026, with service to Merced and infill stations opening by 2030.
Service
, ACE operates four round trips per weekday in the peak rush hour directions – westbound (to San Jose) in the morning and eastbound (to Stockton) in the evening. Trains are scheduled to make the one-way trip in 2 hours 12 minutes, an average speed of . Special trains serve events at
Levi's Stadium. ACE does not operate on weekends or major holidays.
Route

From San Jose to just north of Santa Clara, ACE uses the
Caltrain
Caltrain (reporting mark JPBX) is a commuter rail line in California, serving the San Francisco Peninsula and Santa Clara Valley (Silicon Valley). The southern terminus is in San Jose, California, San Jose at the Tamien station with weekday r ...
main line (Peninsula Subdivision), shared with Caltrain and Amtrak service. From Santa Clara to Stockton – the majority of the route – ACE runs on
Union Pacific Railroad
The Union Pacific Railroad is a Railroad classes, Class I freight-hauling railroad that operates 8,300 locomotives over routes in 23 U.S. states west of Chicago and New Orleans. Union Pacific is the second largest railroad in the United Stat ...
freight lines. From Santa Clara to Newark, ACE uses the
Coast Subdivision, then the
Niles Subdivision to
Niles. From Niles to Lathrop, the line uses the
Oakland Subdivision.
From Lathrop to Stockton, the line uses the
Fresno Subdivision.
The route runs through
Niles Canyon, parallel to the
Niles Canyon Railway,
Highway 84, and the
Hetch Hetchy Aqueduct. The line passes through a long tunnel which cuts off one of the canyon's horseshoes. This tunnel was modified from its original configuration to accommodate intermodal double-stack freight trains. However, this left the track in poor condition, reducing speeds from to in the summer and as low as during the rainy season. The San Joaquin Regional Rail Commission plans to rehabilitate the tunnel.
East of Pleasanton and Livermore, the line runs through the
Altamont Pass
Altamont Pass, formerly Livermore Pass, is a low mountain pass in the Diablo Range of Northern California between Livermore in the Livermore Valley and Tracy in the San Joaquin Valley. The name is actually applied to two distinct but nea ...
on the original
Feather River Route. After crossing the
California Aqueduct
The Governor Edmund G. Brown California Aqueduct is a system of canals, tunnels, and pipelines that conveys water collected from the Sierra Nevada and valleys of Northern and Central California to Southern California. Named after California Gov ...
and the
Delta-Mendota Canal into the
Central Valley, skirting the southern edge of
Tracy. It then turns north between Lathrop and Manteca and runs to
Robert J. Cabral Station in Stockton.
Tickets and fares
ACE fares are distance-based and available in one-way, round trip, 10 trip, 20 trip, and monthly passes. Unlike many of the other commuter rail services on the West Coast, ACE does not have
ticket machines at stations. Passengers are encouraged to use the railroad's mobile ticketing app, but paper tickets can be purchased from agents at all stations except Vasco Road and the Santa Clara Transit Center. Also, unlike many other transit services in the Bay Area, ACE does not accept the
Clipper Card.
Rolling stock
ACE operates
push-pull trains with one to two diesel locomotives and four to eight bilevel coach cars.
Trains typically operate with the locomotive(s) leading westbound and the cab car leading eastbound.
ACE has ordered 17 additional Bombardier BiLevel cars (5 cab cars and 12 coaches). Deliveries were expected to begin sometime in 2021. The entire fleet of Bombardier bilevel coach cars and MPI F40PH-3C locomotives will be rebuilt, overhauled, and repainted to have a matching paint. They will then continue to operate along with the newer Siemens Charger SC-44 locomotives, which entered service in 2020.
References
External links
Altamont Corridor Express websiteSan Joaquin Regional Rail Commission website
{{Authority control
Altamont Corridor Express
California railroads
Passenger rail transportation in California
Public transportation in the San Francisco Bay Area
Public transportation in the San Joaquin Valley Area
Public transportation in Alameda County, California
Public transportation in San Joaquin County, California
Public transportation in Santa Clara County, California
Transportation in San Jose, California
Diablo Range
Fremont, California
Livermore, California
Pleasanton, California
Santa Clara, California
Stockton, California
Tracy, California
Standard-gauge railways in the United States
Commuter rail in the United States
Railway lines opened in 1998
1998 establishments in California