MBTA Commuter Rail recently tweeted out two photos of the rail destressing project. More details about the project here. This also explains the station closure - you really don't want to be near chopsaws and open flames while waiting for your train.
Not an MBTA employee, not a Keolis employee, just a regular commuter (with a scanner and some knowledge of railroad operations). Everything here is my speculation, so up to you to judge the accuracy.