Elected officials in Dorchester and Roxbury are on guard against a plan to provide Commuter Rail service to far-flung Foxborough and worry the plan could signal the end of efforts to provide rapid transit service to Boston's needier communities.

The MBTA is considering a plan that some worry could harm service to the Fairmount Commuter Rail line that runs through Dorchester and Roxbury to Hyde Park. A pilot program awaiting approval by the MBTA would extend the route of several trains per day from Hyde Park to Foxborough, a move the Baker administration says will boost job growth along the Rt. 1 corridor.

U.S. Rep. Michael Capuano told WGBH News he doesn't oppose Foxborough getting more train service, but he wants to make sure it doesn't harm the frequency of the trains on the Fairmount line.

"The more people we can service, the better off we are. But that doesn't mean you service more people at the expense of people who are already being served," Capuano said.

Capuano used $53,000 of his own campaign money earlier this year to give free rides on the Fairmount Commuter Rail line as a promotion to increase ridership. 

The MBTA's board has been considering the plan for months and could vote to approve it as early as Aug. 14. Capuano expects the board will deliberate and come to a fair compromise with the Fairmount Community.

"If they vote to just do it and ignore the community that's currently being serviced, we'll have a difference of opinion. We'll have a problem," Capuano said.

An MBTA presentation to the board Monday said the Foxborough proposal will continue to service the Fairmount Line as it is served today.

"The operation of the Pilot does not reduce existing Fairmount Line service, stops, or frequencies," the presentation said.

Dorchester Rep. Evandro Carvalho told the MBTA's board at their Monday meeting that extending the Fairmount line beyond its urban focus doesn't make any sense, given that the goal of many of the line's backers is to become a rapid transit line carrying Bostonians from outlying neighborhoods to South Station.

"It wants to make it more of a Commuter Rail line. Extend it far beyond the city limits which it's primarily being used for now, to go to Foxborough and other places. That means the frequency is not being taken care of," Carvalho told WGBH during an interview in his Dorchester district.

The Dorchester Democrat is worried that extending the line will introduce delays to the already limited Fairmount schedule. Carvalho also worries that partially dedicating the Fairmount line to other uses will prevent further investment in achieving more rapid service for what he called a "transit desert" that relies mostly on bus lines.

"They're saying that the little that already have, we're going to take it away and serve communities that - I don't know that community that well but it seems that they're certainly better off than us," Carvalho said.