Two legislators from Boston make the case—unconvincingly—that the Fairmount Line should be extended to the Seaport, rather than its current terminal at South Station. This makes no sense at all. The point of the Fairmount Line—and especially making the Fairmount Line more like a rapid transit line—is to connect to the rest of the MBTA's system. You can do this at South Station, easily: trains terminating there allow for an easy connection to the Red Line and Silver Line, outbound Commuter Rail to Back Bay, as well as putting passengers right in to the Financial District. While a terminal in the Seaport (but really by the Convention Center, probably, which would be as far from much of the Seaport as the South Station; anything east of A Street is closer to South Station, and a much more pleasant walk) would provide a one-seat ride for anyone working in the Seaport, it would connect to nothing, except for the already oversubscribed #7 bus there.
Add in numbers and they speak for themselves. According to the City of Boston, there are 27,000 workers in the Seaport District. This is a large number, but it compares with 222,000 in Downtown or on the Red Line (i.e. near Charles/MGH) and another 80,000 in Back Bay. While the Seaport district may be "booming" right now, the rest of the City has been "booming" for decades or centuries. There are ten times as many jobs within a 10 minute walk or transit ride of South Station as there are within a 10 minute walk or transit ride of the Convention Center. These data also doesn't include much better access to Cambridge, which amounts to another 100,000 jobs, most of which are an easy ride from South Station on the Red Line.
If you connect the Fairmount Line to the Seaport, you do provide a one-seat ride to these 27,000 jobs (although you wind up further from them than you'd really want), but you lose one-seat access to 180,000, and an easy connection to another 100,000 on top of that. This is just a terrible idea. It decreases access from the Fairmount Corridor to the rest of the system by dropping people in the middle of a concrete wasteland, with very few connections to make. If you work in the Seaport, lucky you. But this is the case for only 2% of Dorchester residents and 1.3% of Mattapan residents. 15%—ten times as may—of people living in these communities work downtown. As for non-work destinations, unless you're going to a convention or the ICA, there's really no reason to use the line. So ridership would be very light. And if you do need to get to the Seaport? From South Station, there's a Silver Line or #7 bus every couple of minutes, plenty of Hubway bikes, and it's not that bad a walk if the weather is nice.
Now, if the line somehow had a great connection to the Red Line at Andrew or Broadway, it might be a bit more feasible. Still, you'd be making passengers headed downtown (the majority of riders) transfer, and you'd be making them transfer to the Red Line at its peak load point, rather than at South Station, where many passengers from the south are getting off. But you don't: these transfers would require a walk of several blocks, which most people are unlikely to make.
What about a split terminal, with some trains going to South Station and others to the Seaport? It would work in theory, but not in practice. There are only two railroads in the country which have split terminals, both of which are in New York. This is partly due to the geographical fact that New York has two main employment centers several miles apart: Midtown and Downtown. The Long Island Railroad runs most service to Penn Station, but some trains to Atlantic Avenue (with an easy subway connection to Downtown) and a few to Long Island City. However, ridership on the LIRR is 338,000 daily, triple the entire MBTA Commuter Rail system, and nearly all trains provide an easy, cross-platform transfer at Jamaica. New Jersey Transit runs most trains to Penn Station, but some to Hoboken, but again, Hoboken provides an easy transfer to Downtown via the PATH Tubes, and transfers can be made easily at Newark. Fairmount, on the other hand, has never had and will never have anywhere near the traffic required for such a system to work, nor is there a logical transfer point. It needs to have one terminal, and the only logical terminal—until and unless the North-South Rail Link is built—is at South Station.
Investment in Fairmount, as has been posited by this page in the past, should focus on two features (in addition to more frequent service, which should be a given). First, it should have all-door boarding and not require conductors to collect fares (this could be solved easily with proof-of-payment fares or by installing fare gates on a platform at South Station). Second, it should be converted to electric operation, to allow for faster travel times and less noise and pollution in the neighborhoods it serves. Either way, it should terminate at South Station. (What to do with Track 61? A connection from the Red Line at Andrew to the Seaport makes more sense.)
The promise of Fairmount is that it could provide a quick, frequent trip from Dorchester and Mattapan to Downtown Boston in half the time—or less—of the current bus-rail transfer along the route. The number of people who desire a trip to the Seaport is small. Even if employment there doubles, it will still be a drop in the bucket compared to Downtown, and all the jobs you can get to with an easy transfer to the rest of the subway system. I think Rep Collins and Sen Dorcena-Forry's hearts are in the right place: they want Fairmount to provide better service to their community. But it should provide service to where there are more jobs and better connectivity: South Station.