The MBTA opened a new, $15.9-million commuter rail station Monday at Talbot Avenue in Dorchester along the Fairmount Line. The station is the first of four along the Fairmount commuter rail line that are being built, officials said. The 9.2-mile line had four other stations before Monday. The railway runs from South Station through Dorchester, Roxbury, Mattapan, and Hyde Park, where it ends at the Readville Station. Among the MBTA’s dozen commuter rail lines, the Fairmount is the shortest in distance, has the lowest ridership, and is the only one that exclusively serves Boston, according to the transit agency. The T’s acting general manager, Jonathan Davis, said each of the new stations, along with related improvement work along the line, will help boost ridership, providing an alternative for commuters who live and work along the Fairmount Corridor. “It’s a very densely populated corridor with a lot of heavy traffic, and this will provide a more convenient and faster way to get around,” he said by phone Monday.