Should the Fairmount Line be a traditional commuter rail service, running from the city to a distant suburb and back again during rush hours?

Or is it better used as a rapid-transit service that provides a faster-paced link to downtown for Boston’s minority neighborhoods?

Those two visions would seem to be in conflict, and the differences soon may play out for real. The MBTA is planning to extend Fairmount service to Gillette Stadium in Foxborough in 2019.

Meanwhile, the administration of Boston Mayor Martin J. Walsh is working on a pitch for additional daily service within the city — a step toward the decades-long goal of using the Fairmont Line as a rapid-transit connection for a large section of Boston that has no subway access.

“The future of the line is not set in stone,’’ said Dorchester state Representative Evandro Carvalho, who has filed legislation seeking more frequent Fairmount service. “The line, if it extends to Foxborough, could derail the plans for more rapid transit. But not necessarily: It kind of rejuvenated the push to get the rapid transit done as well.’’

Currently the Fairmount Line runs between South Station and Hyde Park, through Mattapan and along the Dorchester-Roxbury line, with trips at least every hour.

The only commuter rail line that operates entirely in Boston, it had a rash of train cancellations in 2016 that led to a civil-rights investigation. It has since recovered and is now the T’s most reliably on-time commuter line.

But advocates along the Fairmount worry that the upcoming Foxborough service could interfere with that progress.

The extension to Foxborough, partly financed by New England Patriots owner Robert Kraft, will run as a one-year test starting in 2019 after it was approved by the T’s governing board in August. Eight of the nine daily trips will originate or end in Foxborough using Fairmount trains, and will also make stops in Dedham, Norwood, and elsewhere.

Any breakdown or delay near Foxborough could disrupt trips in Boston, advocates worry. And some see the pilot service as a more existential threat: if it is made permanent and expanded, they say, the Fairmount Line may be reoriented to become a “Foxborough’’ line, with the scheduling needs of a more extended line out to the suburbs squelching their ambitions for more frequent service just within Boston.

“If we’re just going to make it into the Foxborough line, I think our long fight would be for naught,’’ said Pamela Miles of the Fairmount Indigo Transit Coalition.

The Walsh administration offered measured support this summer for the Foxborough service but noted concerns about disruptions to Fairmount service. Then in September, Walsh said city officials are developing a pilot project for the MBTA to run more service among the line’s Boston stations.

It seems like a tight needle to thread: adding service through Boston while extending the line into suburbia. But Vineet Gupta, Boston’s transportation planning director, said the city’s idea — to run more trains during off-peak periods — could coexist with the Foxborough service.

The Walsh administration is expected to preview its ideas for transit service Monday, including on the Fairmount Line, when it presents a long-term plan for transportation options to the MBTA board.

City officials will later propose a more formal, detailed pilot project for Fairmount service, though there is no set timetable for that.

MBTA officials declined to provide details about their plans for the future of the line.

Spokesman Joe Pesaturo said officials “understand the city of Boston’s desire to see more frequent service on the commuter rail line, and we will continue to work with city officials and elected leaders in an effort to grow ridership and bring even more improvements.’’

The quest to increase Fairmount frequency goes back decades. After the elevated Orange Line along Washington Street was torn down and relocated in the late 1980s, advocates envisioned partly replacing it with a different type of high-frequency commuter rail service.

It was also often mentioned as the central part of a larger “Indigo Line’’ network that could include branches to Allston and Cambridge.

For many in Boston, the idea behind increased Fairmount service is to better connect low-income and minority neighborhoods — not just to downtown but to each other. Most commuter rail lines operate quite differently: on a set schedule, as the vast majority of riders get off downtown in the morning and leave that station at night, with only infrequent midday service.

There is evidence Fairmount passengers don’t treat it like a typical commuter rail line. It has a higher percentage of off-peak passengers and of passengers who disembark at non-downtown stations than other rail lines, according to a 2017 study from the nonprofit Boston Foundation.

The study also found Fairmount ridership tripled between 2012 and 2016, though at 2,260 passengers a day, it has the lowest of the 12 commuter rail lines.

The ridership counts have raised questions about the demand for more service. T officials cited the Fairmount’s low ridership when they opted to extend the service to Foxborough, arguing the trains have room.

Advocates, however, say there may be more demand, if only it ran more often.

“It’s a chicken-and-egg kind of problem,’’ said Keith Mahoney, a spokesman for the Boston Foundation. “How do you increase the frequency if the ridership isn’t there, and how do you increase ridership if the frequency and reliability aren’t there?’’

The MBTA says it has taken some action to boost service. The final station at Blue Hill Avenue in Mattapan is scheduled to open in 2019, the fourth new station this decade. Fares were lowered to the same price as the subway in 2011(now $2.25 with a CharlieCard),and weekend service was introduced the next year.

On weekdays, the service already runs more than 40 trips, with off-peak service every hour — more common than most commuter rail lines.

But there have been setbacks. The Baker administration shelved the plan by former Governor Deval Patrick to purchase a new type of train that can make quicker, more frequent trips.

Some ideas to improve Fairmount service have little to do with frequency, said Rafael Mares, a Conservation Law Foundation vice president.

For example, he said, passengers should be allowed to use plastic CharlieCards to pay fares, because it would simplify payments for passengers who also use neighborhood buses; the cards can now be used on buses and subway rides but not on commuter rail.

The MBTA, however, said equipping conductors with technology to process CharlieCards would be expensive. Moreover, the T plans to launch a new fare collection system by 2020 to give passengers more options to pay fares across all modes — including the Fairmount Line.

One cheap and easy idea to increase train frequencies was included in the Boston Foundation study: reroute off-peak Franklin Line trains from Amtrak’s Northeast Corridor onto the Fairmount tracks to stop at those stations instead.

It would cut the wait time between midday trains but would still be far from rapid transit.

That vision is included in Boston’s long-term transportation plan, which proposes eventually running trains every 5 to 10 minutes. Yet those wild dreams seem harnessed by reality, as the city’s plan notes a major roadblock: “the MBTA’s already overburdened financial constraints.’’

Adam Vaccaro can be reached at adam.vaccaro@globe.com.