Wicked and BandA, I think you're both missing my point. I am not saying that it would be quick or easy to start running the Fairmount line like an RT line. I am saying that for around the same amount of money, planning, construction, and changes in the way politicians and passengers imagine the line that it would take to change the Fairmount Line into an RT line, it could be made into an FRA rail line capable of offering service similar to RT service. I am saying that there is no big reason for an argument between those who want RT-type service and those who want an emergency or construction-related detour route from South Station to Readville. I'd be interested in hearing if you think I'm wrong about that, because I am no kind of expert, just someone who's seen a whole bunch of different kinds of trains and platforms and rights of way and knows that there isn't just one kind of rapid transit and one kind of commuter rail that are very different from each other, but rather a continuum of overlapping types of ROW, equipment, and service patterns.
Is anyone likely to come up with the money for either making it an RT line or making it an FRA line capable of running RT headways? Probably not, partly because even in the most public transit friendly parts of the US our political culture treats roads as a necessity and transit as a luxury.
Is it a good idea? Maybe; I don't know where people from Hyde Park and Uphams Corner and whatnot want to go and what the best way is to get them there.
Would it be better than turning the line into an RT line that regular trains can't use? Yes.