PORTLAND, Maine — Amtrak’s Downeaster is celebrating its 10-year anniversary this week, capping off a year in which annual ridership surpassed half a million passengers, Amtrak launched its eTicketing pilot program and workers made progress on an expansion of service north to Freeport and Brunswick that is to begin in the new year.

Ridership has doubled over the past six years, and the number of passengers topped 500,000 for the first time in 2011, Amtrak said. All told, more than 3.5 million passengers have ridden the train.

“It’s been a success by every measure and we’re looking forward to the expansion of the service to Brunswick,” said Stephen Gardner, Amtrak’s vice president of Northeast corridor infrastructure and investment development in Philadelphia. “This is a corridor to watch.”

The Northern New England Passenger Rail Authority, which oversees the Portland-to-Boston service, has a string of events planned leading up to Thursday’s anniversary.

Ten years ago, however, nobody knew whether the service would succeed when the first regularly scheduled train departed from Portland on the cold, rainy morning of Dec. 15, 2001.

Back then, some New Hampshire officials were openly skeptical of the prospects of the first Portland-to-Boston passenger rail service in more than 35 years. These days, New Hampshire still doesn’t contribute any operating costs, but the value of the train service speaks for itself.

“It’s hard to argue with the numbers,” said Bill Boynton, spokesman for the New Hampshire Department of Transportation. “It’s done very well.”

Ridership took off starting in 2005 thanks to increased frequency of round-trip service, track improvements that have shortened transit time and gas price spikes.

Over the past year, the Downeaster marked several milestones that included launching Amtrak’s eTicketing pilot program that allows passengers to print their tickets at home. Amtrak also boosted its free wireless network for riders who want to use their laptop computers and other digital devices.

For its 2011 fiscal year, ridership reached 509,986 passengers, and those numbers will continue to grow when the Downeaster service expands northward to Brunswick, said Patricia Quinn, executive director of the Northern New England Passenger Rail Authority, based in Portland.

A $38.3 million track extension is due to be completed in the new year, allowing stops in Freeport and Brunswick. That will mean another 36,000 annual passengers initially, then an additional 50,000 passengers once service expands to five roundtrips per day, Quinn said.

Existing stops between Portland and Boston are Old Orchard Beach, Saco and Wells in Maine; Dover, Durham and Exeter in New Hampshire; and Haverhill and Woburn in Massachusetts.

Lionel Caron was one of those who rode the first regularly scheduled train to Boston. He said he never had any doubts that the service would succeed, especially after gas prices rose. With the cost of gas, and the cost of parking the car in Boston, the Downeaster seems like a bargain, he said.

Caron said he enjoys the low-key vibe of the train, which he has ridden several times. And he stands to benefit from the new station in Brunswick, since he lives about 12 miles away in Lisbon.

“If you fly, you don’t see much. If you take a train, you can look out the window and relax,” he said. “You can’t do that on a four-lane highway either.”

It was no small feat to get the service running.

It took 13 years and more than $60 million in public funds for track upgrades and equipment and countless hours of negotiating among various parties to get passenger trains running for the first time since 1965 on the 116-mile route between Portland and Boston.

“It does call for a celebration,” said Dana Connors, former Maine Department of Transportation commissioner who now serves on the rail authority’s board.

To mark the anniversary, the Northern New England Passenger Rail Authority is holding a pizza party for Amtrak workers on Monday and a luncheon for volunteers on Tuesday. On Wednesday, Amtrak and rail authority officials will ride the train from Boston to Portland.

And on Thursday, rail authority board members, transportation officials and other dignitaries will participate in an anniversary ceremony in Portland that coincides with the arrival of a Toys for Tots train carrying nearly 150 schoolchildren from Berwick, Maine.

Starting Dec. 15, fares are being reduced to $15, the lowest published fare for a one-way ticket in coach.

Wayne Davis of TrainRiders Northeast, which lobbied for a dozen years to make the rail service a reality, tries to avoid saying “I-told-you-so” but the reality is that he did.

“It’s exciting to think it was 10 years ago. Son of a gun,” Davis said. “Everything we ever said came true.”