(1) The Green Line has a minimum turning radius which is simply incompatible with any decently-sized heavy rolling stock, especially in the Tremont Street subway. Off the top of my head, there are hairpin turns south of Bolyston and on either side of Park Street, and the sharp turns between Science Park and North Station would certainly be a bottleneck as well. Much of the existing Green Line would require realignment in order to operate heavy rail at any kind of reasonable capacity. Not cheap.

(2) Running high-frequency heavy rail along the Green Line’s grade-level rights-of-way west of Kenmore and south of Symphony would be nothing short of unacceptable. This type of rapid transit service essentially requires grade separation to be efficient and safe, whether it be elevated or subway. Elevated is ugly, subway is expensive — pick your poison. (Amusingly, this requirement completely solves the problem of securing current ground-level stations — by replacing them.)

(3) The E branch joins the main line at a level junction west of Copley. This setup already causes irritating delays as westbound E trolleys cross the eastbound main line track at grade; this would be a major source of congestion with frequent heavy rail, likely necessitating the construction of a flying junction, at no small cost.

(4) The option of mixed light/heavy rail service leaves much to be desired. The forced transfer at Kenmore would be incredibly unpopular among a ridership that would remember the past through-running service. This model might currently work at Ashmont, but Kenmore would be handling orders of magnitude more transfer traffic. And mixing ATC heavy rail with (presumably) manually-operated light rail between Copley and Park Street seems like a suboptimal solution. I am aware of existing systems that run trams/trolleys on mainline railways, and perhaps this service pattern would serve the Green Line well, but it’s certainly not a given.

(5) Last but not least, there is the matter of rolling stock maintenance. I believe yards of varying size currently exist at Riverside, Boston College, Cleveland Circle, and Lechmere, but none of these are fit for storing or maintaining heavy rolling stock. Perhaps Riverside Yard, the current major Green Line maintenance facility, could be significantly upgraded and expanded. Or maybe a whole new railyard would have to be built. Or maybe the Green Line could be connected to another line so that its rolling stock could be maintained at Wellington Yard (Orange Line), Cabot Yard (Red Line), or Orient Heights Yard (Blue Line). None of these options are straightforward… or cheap.

I love the idea of converting the Green Line to heavy rail, but it wouldn’t be easy. I can see why the MBTA is trying cheaper measures to improve service, although the results are debatable.

P.S. There are several other New York lines which I bet exceed Green Line crowding. The Queens Boulevard express and Broadway-Seventh Avenue lines come to mind. Without solid data, though, this is merely speculation.

I think in an ideal world, we would see some combination of more tunnels and elevated tracks. I think the C line ought to be left the most alone due to the less urban feel of Brookline. The B line could be tunneled straight out to Packard’s corner, and then run elevated until Washington St., and the E line could be tunneled until Brigham Circle.