Altamont Commuter Express train pulls into the Stockton station. If it isn’t derailed by Proposition 6, the ACE train will be coming to Modesto within two years. It will reach Merced soon after. aalfaro@modbee.com

As the ACE Train pulls into the Santa Clara station, the conductor pops out – and begins apologizing for his train.

“I’m sorry, but this is not the Amtrak!” he bellows, loud enough to be heard by everyone on the long platform

“And this is not Caltrain! If you want the Caltrain to San Francisco, do not board this train!” he yells.

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The ACE Train uses some of the same tracks but doesn’t go the same places as Amtrak and Caltrain.

The ACE Train is important because of what it is not. It’s not a service that operates around the clock, like LA’s Metro. It’s not charming and tourist-friendly, like San Diego’s trolley, and it doesn’t connect our fanciest precincts and companies like BART. It’s not expensive to build, like the high-speed rail project. And it’s not losing riders, like so much public transit these days.

Here’s what the ACE Train is: a real, live, and unappreciated story of successful transportation in California. And while its story is modest and narrow, it’s planning expansion in ways that – if Californians can move past the brain-dead populist politics of the gas tax – should point the way to a future in which Californians can move around more easily.

The Altamont Corridor Express is modest. Its service consists of just four round trips each weekday – reflecting the fact it shares tracks with Union Pacific. ACE sends four trains from Stockton to San Jose, via the East Bay in the morning, and four trains back from San Jose to Stockton at the end of the day.

The ACE Train works because it is pure commuter rail. Every morning ACE takes residents from Livermore, Lathrop, Tracy and Manteca to jobs in the East Bay and Santa Clara, then returns them home in time for prime time TV. It keeps them off the madness-inducing parking lot that is I-580.

ACE started 20 years ago backed by a joint powers authority and funded by a sales tax increase in San Joaquin County, whose residents suffer from some of America’s longest commutes. ACE ridership has doubled in the last six years to 5,000 a day, 1.3 million annually. It’s one of the fastest-growing train lines in the country.

By 2020, the service is scheduled to expand in two different directions at once. One branch will head to Sacramento, with stations in Lodi, Elk Grove and Natomas, ending with a shuttle to Sacramento Airport. The other extends south to Modesto and Ceres before eventually connecting to Merced.

In combination with expanded service on Amtrak’s San Joaquin line, ACE will form a triangle between three regions – Bay Area, Capital Region, and the San Joaquin Valley.

It will also put ACE at two of the most important new transportation hubs of 21st-century California. The first is San Jose’s Diridon Station, which already links Caltrain, Amtrak and Santa Clara’s VTA light-rail system. High-speed rail’s first phase would end there, connecting to a massive new Google site.

The second hub is downtown Merced, which would be both an ACE terminus and a stop on high-speed rail.

Unfortunately, this second extension – to Modesto then Merced – is endangered because it is funded by the controversial gas tax increase that would be repealed by Proposition 6.

The gas tax is a statewide political battle, but the geographic center of the fight is the ACE corridor. Two lawmakers from there – Senator Anthony Canella, a Republican from Ceres, and Assemblyman Adam Gray, a Democrat from Merced – voted in favor of the gas tax in exchange for $400 million for the ACE expansion.

If Prop 6, which is popular among Republicans, passes, the ACE expansion is threatened.

Democratic congressional candidate Josh Harder has cynically come out in favor of Prop 6, even though it would hurt his hometown of Modesto, to create political problems for the area’s incumbent Republican Congressman, Jeff Denham, who is heavily funded by transportation lobbies. Denham was previously such a champion of ACE that he held a town hall on the moving train.

Now Denham is trying to play it both ways: He has quietly endorsed Prop 6 to appease his tax-hating GOP base, while refusing to give the measure money or vocal public support.

Riding the ACE is less complicated than voting on it.

I boarded the train in San Jose, and marveled at the big crowds that embarked at the next two stations. The first, Santa Clara, has a shuttle bus to the San Jose airport, while the second, Great America, is next to the 49ers’ new stadium. The platform there was mobbed with employees of Cisco and other tech firms.

By the time the train passed a beautiful stretch along the southeast edge of the Bay and stopped in Fremont, there was no longer a seat available. A group of Cisco engineers had a business meeting around one table. At the Pleasanton stop, new riders, who shuttled over from the BART station, squeezed on.

The train slowly emptied over the next four stops – Livermore, Vasco Road, Tracy, and Lathrop/Manteca – as people poured into jammed parking lots. Some had bicycles and rode off on them. ACE riders told me the traffic jams getting into these lots is the most difficult part of their trip. The crowding might get worse: New construction of housing and retail was visible near most stops.

I heard some complaints about ACE’s Wi-Fi, and the cost of the train (monthly passes can run more than $300), but the trip is still cheaper and easier than driving.

My train was mostly empty by the time I got to lovely Cabral Station, on the edge of Stockton’s downtown. From there, I walked to a dinner at Angelina’s Spaghetti House. And I didn’t have to hurry – the ACE had arrived five minutes early.

Joe Mathews writes Connecting California for Zócalo Public Square.