One problem would be that it ran in the street, not in its own right-of-way, and I don't think that would fly today, but the bigger one, the thing that killed it, is the Newton I-90 interchange. Before the Pike the A branch crossed the railroad tracks on the Washington Street bridge, which carried traffic in both directions. After the Pike, that bridge was one way northbound for cars, but two way for the trolleys. In theory, so long as traffic followed the lights vehicles wouldn't get stuck on the bridge blocking the trains when the southbound trains had the green. In theory.

I have concerns about West Station too, because it's very close to Boston Landing, and having stops super close together slows things down.