The whole point of modernization and organization-before-electronics-before-concrete thinking is that commuter rail should run frequently at all times of day, since it's cheaper to implement the necessary changes than to build new bridges across rivers and railyards. Working-class immigrants don't care whether the train has a green stripe on its side and terminates in Chelsea, or has a purple stripe and continues into the outer suburbs; they care that the train comes frequently and doesn't require them to pay higher fares than buses. So do middle-class native-born office workers, which is why the D branch has so much more ridership than the parallel commuter lines.But these are dense, inner-core communities that include large immigrant populations that don't necessarily work the 9-6 office hours downtown that CR schedules tend to serve. Rapid transit is preferable. The ROW is wide enough for both CR and GL - albeit, and I agree with you here, it would require a new Mystic Bridge and some intensive engineering to bring the line down to Sullivan and on alignment to Lechmere.