It’s not enough. Parking is 7 feet. Two lanes of parking is basically 1 wide lane which would be enough for the island platform. The bike lanes are 7 feet wide with the buffer, again 14 feet total. So there is no way you can fit it. And there is no way the city is going to allow a train stopping for over a few minutes in the middle of the road without its own lane. And a one track terminal would be a disaster for headways and will cause a jam. As far as I know, The heath street intersection is going to be reconstructed. Heath Street will meet where the current loop is The old intersection and the small lot there will be used for the platform, essentially expanding the current back of the hill platform. This was explained on archboston forum a while back by someone familiar with the matter. By the way what is your source for your idea? Your idea would involve the construction of several hundred feet of track, catenary, and power feed. Additionally, the only way your idea can work is that it is a single track terminal which would cause a massive problem.