That's never going to be a viable routing for commuter rail service because of number of grade crossings that would be speed-restricted even at upgraded track speeds because of the curves. And that even includes someday elimination of the Mass Ave. grade crossing, which MIT wants to do by burying the roadway Broadway-under-Harvard Yard style on that block for a pedestrian mall with tracks/Urban Ring remaining on the surface. The route would definitely work as light rail because of the shorter consists and faster acceleration/deceleration. But that'll be a slow trip in a loco, messing up multiple streets at a time as it plows through. Some of the Amtrak moves already mess things up pretty good Broadway and Main when they choose to go through at 5:30 or 6:00 on a Friday.But if they installed "real" grade crossings with actual gates, might the speed on the line be good enough to make the trip worthwhile? Cambridge, Binney, Broadway, Main, and Mass Ave would all be made way safer with gates. That's 5 crossings that could then be taken faster than the CRAWL at which they are taken these days. I imagine even a steady 20-30 mph through here would do wonders.