Let's see: Proposals include tunneling to Brookline Village, setting up a loop to the Riverside, and preserving Heath rail service for the JP VA which may, itself, have a limited life expectancy. Granted, from Brigham Circle to Brookline Village is not that far either in the dictionary or by measurement, but whether by cut-and-cover or deep-boring, tunneling will be immensely disrupting to the area and thus politically difficult. But, someone might well say, deep-boring means that traffic can continue to run while the tunnel is being formed. True, and adding to the traffic will be all the truck-trips carrying out spoil and bringing in concrete, ties, rails, and all the other items necessary for the tunnel and stops along the way. One other engineering complication will going under the Muddy River. If the connection with the Riverside Line is made, that will have been with done its own and additional complications and costs. Could it be done? Sure. Will it be done? Not very likely. Given the T's finances and the related shift away from street-running light rail, what will in all probability happen is increased reliance on rubber wheels instead of steel to get people to and from both rapid transit and off-street light-rail.
Having grown up near the intersection of Centre and South Huntington and having dropped fares into the boxes and student tickets into the hands of workers of center-doors, Type 5s, and PCCs of various types, including Texas Twisters, I do miss the squeal of rotating steel on sharply-curved steel, but until and unless JP gets Disneyfied, I can't see street running happening. No offense, but absent some WPA-like project of the sort that extended the Huntington Subway to its present surfacing, tunneling to Brookline Village also seems Fantasy Land.
"A gray crossover is definitely not company transportation."