I love the Minute Man and ride it monthly if not weekly. I don't see how it existing would block a major transit expansion though. As the Wikipedia article points out, the MBTA still owns the ROW:

> The property is currently owned by the Massachusetts Bay Transportation Authority and leased to the municipalities through which it passes on an interim basis.[citation needed] The MBTA at one point planned to use this right-of-way to extend the Red Line to Arlington Center and Arlington Heights.[10]

And while NIMBYs at one point blocked the Red Line extension on racist grounds (see this article for the deets) it's not that hard to bypass NIMBYism. We simply tell them to pound sand and commit to funding the entire extension in one go. The State can do whatever it wants with the ROW it owns. It can even play hardball and start denying grants and funding to towns who refuse to play nicely with transit. Watch how quickly Arlington agrees to a Red Line extension if it finds out the millions it gets in State Grants go away.

I also don't see why the Minute Man couldn't be repaved once a Red Line extension is completed. That would serve nicely for MBTA access when needed, and temporarily pathway detours can be setup using parallel streets. Now if we were talking about using it for Commuter Rail that would be an issue, as the Minute Man has dozens of at grade crossings to deal with plus not enough ROW to handle double track trains and a pathway at grade.

The Blue Line to Watertown also isn't duplicated if it's done on the Cambridge side of the Charles using existing roadway ROWs. Motorists would be peeved at losing access temporarily to a major roadway, but there's already Storrow and i90 on the other side plus plenty of alternative routes to use. We could even loop the Blue Line back around via the Watertown/Cambridge Greenway. Or run a branch of the Red Line down that way instead.