ACE Rail, the Altamont Commuter Express (a new name of Altamont Corridor Express is presently being phased in, appearing on tickets and a few new signs) is a Commuter Rail Line that makes four weekdays only peak-direction only, 86 mile round-trips (westbound in the morning, eastbound in the evening) from Stockton in the San Joaquin Valley over the Altamont Pass to the Tri-Valley and through Niles Canyon to Santa Clara and San Jose. Riding ACE through Niles Canyon and up the Altamont Pass (by wind farms) is extremely scenic. The sprawl of Silicon Valley means that the system also includes an extensive shuttle bus operation (particularly from the Great America Station) to get ACE passengers from trains to their jobs. The line doesn't connect directly with BART but Wheels bus route 53 provides express transfers from the Pleasanton Station to the West Dublin/Pleasanton BART Station.

ACE Rail on occasion but quite rarely (not to the same degree as other weekday only commuter rail systems like Sounder) runs special weekend trains from Stockton to events, almost every summer there is a special ACE package and round-trip to the Great America Theme Park (the first ran in 2001). Other events to receive special ACE weekend service have included the Stockton Ports (an A's A affiliate) sponsoring a day trip (not more than one per year) to the Oakland Coliseum (with ACE reversing in Fremont and running along the Capital Corridor to reach the Coliseum Station) but this hasn't occurred since 2007-2007.

The system uses Push-Pull Trainsets of Bombardier BiLevel Coaches and F40PHI Locomotives. Normally trains are made up of 6 or 7 cars with a locomotive and run in pull-mode on the AM westbound runs and in push mode for PM eastbound runs. Trains currently use a UP Maintenance yard south of the Stockton Station although a new dedicated ACE yard and maintenance facility is under construction just north of the Stockton ACE station. Trains use a small yard just south of the Tamien Caltrain station (running light from San Jose Diridon past this station) for midday layovers between the single round-trip each trainset makes. Herzog Transportation Services is the contractor for the line and has been since ACE began operations under the San Joaquin Regional Rail Commision.

All trains have the usual tie-downs for bicycles, and stations include LED signs that use the same system as Amtrak California listing the train number (#01), destination (SKT or SJC), and ETA. ACE trains even appear on the same displays as Capital Corridor and San Joaquin Trains at the stations shared with those routes. Surprisingly ACE Rail is the only commuter rail line to share these displays (the shared Metrolink and Coaster stations have separate displays for the commuter trains and Pacific Surfliners).

Ticketing:
Since ACE began in 1998 it has had a most unusual ticketing system. It is POP like most modern commuter rail lines except passengers are asked to wear their ACE tickets (one-way, round-trip, and monthly passes are normal sized, 20-trip tickets are extremely long strips) on their person (some passengers affix them to a handbag, briefcase or shoulder bag) in special transparent holders that are supposedly provided by ACE (your webmaster when photographing this section and buying two ACE tickets never received one). All platforms include validator stamping machines for one-way, round-trip and 20-trip tickets but all ticket sales at stations are handled by humans. Passengers can also buy instant tickets on-line that must be printed out. Ticket agents aren't stationed at every station for every departure and Santa Clara (Downtown) and Vasco never have any ticket agents (see the individual station pages for exact ticketing information) or any in station ticketing options. All passengers at these two stops must purchase their tickets in advance or on-line. The website has notes of "No ticket sales onboard the train as of April 4, 2011." The tickets themselves are all pre-printed and oversized (a one-way ticket doesn't fit well in a wallet) pieces of paperboard that are a strange shape and look old-fashioned. Since 2000 ACE has had a loyalty program for monthly pass holders, buy 11 months of monthly passes, get the 12 month free! To redeem you need to use the old fashioned method of mailing in the previous 10 months plus a photocopy of your current month and then next month's will be sent free.

Amtrak also sells tickets as a Thruway Connection on evening ACE Trains to connect with San Joaquin trains in Stockton. Amtrak Thruway buses provide connecting service from San Jose to the San Joaquin at other times. All four morning ACE trains depart Stockton too early (between 4:20am and 7:05am) for any connections to Amtrak Trains so all westbound San Joaquin connections are via only buses. While waiting for an ACE train I helped a very confused passenger who had been told that over the phone by an Amtrak agent that she was waiting for a bus to take her Stockton that no, the ACE Train Thruway connection is a train. The ACE Thruway connection is even ticketed as an eTicket although ACE crews don't appear to have Amtrak.

History and Service Levels:
ACE started out as an extremely small operation on October 19, 1998 to serve the booming Tech Industries in Silicon Valley. Initial service was just two peak-direction round-trip trains per day using only 8 Bombardier BiLevel Rail Cars with a total of 1,100 seats. Trains stopped at every station except for Santa Clara Downtown that was added later. By September 8, 1999 (the first day ridership in each direction passed 1,000 passengers) the line was at capacity with ridership over 1,000 at 1,077 and reached the capacity of the two trainsets.

• February 2000 — A third new morning turn-back train from Pleasanton to San Jose. The first trainset arrived in San Jose and ran non-stop (accepting passengers) to Pleasanton and then stopping at Fremont and Santa Clarita on its way back. No similar short-turn train was made during the PM Rush Hour. Also an evening bus trip from San Jose to Stockton was added and is only available to 20 trip and monthly passengers. The Pleasanton and Tracy stations are most popular for bording passengers, seventy-five percent of riders get off at Santa Clara for shuttles. This AM reverse-peak trip to Pleasanton is the only reverse-peak train ACE has ever operated.
• March 5, 2001 — A third round-trip enters service running only from Lathrop/Manteca to San Jose Diridon. This trip is the last train in each direction. This replaces the additional AM rush hour Pleasanton round-trip that is discontinued. All trains also begin stopping at the Santa Clara downtown station. Santa Clara station has always been a Caltrain stop and is the only infill stop to ever be added to ACE Rail.
• From October, 29 to November 9, 2001 — service is discontinued south of Fremont with a bus bridge due to Union Pacific Cayote Creek Construction Project, to compensate riders ACE Monthly Passes are discounted 50%
• Effective January 6, 2003 — Third round-trip (last train in the morning and evening) reaches Stockton with a connecting Stockton Solution Shuttle
• August 9, 2005 — All 3 trains now originate and terminate in Stockton. Train service to the Santa Clara Downtown Station is discontinued because of the major construction project there building a new platform and underpass and realigning the tracks. One southbound and two northbound connecting to San Joaquin trains Amtrak buses are now listed in timetables as available to ACE Train riders, these buses are jointly funded by Caltrans (for the San Joaquin Trains) and ACE.
• Starting August 28, 2006 — ACE adds a midday round-trip. It leaves Stockton at 9:30am and returns from San Jose at 12:05pm. The train is partially funded by Caltrans to absorb traffic from I-215 that is starting to undergo a three year construction project, and also Amtrak California that will connect passengers via this train to San Joaquin trains #711 and #716.
• November 2, 2009 — The midday round-trip, only on average 50 passengers were using the train in each direction is discontinued. ACE is back to running just three round-trips per weekday. Ridership and funding is also decreased because of the Great Recession.
• May 2012 — Train service to Santa Clara Caltrain is finally restored after the lengthly project to build and underpass is completed
• October 1, 2012 — A fourth daily train added to the schedule again, now its another later rush-hour round-trip in both directions with 4 trains now leaving Stockton between 4:20am and 7:05am (arriving San Jose between 6:32am and 9:17am), return trips depart San Jose about hourly between 3:35pm and 6:38pm arriving in Stockton between 5:47pm and 8:50pm.