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’s 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.