The Wikipedia article and citations for the Mass Central Rail Trail answers some of the questions in this thread:
https://en.wikipedia.org/wiki/Mass_Cent ... _-_Wayside
Protect Sudbury lost the 2nd petition to the STB. They also lost the 2 Massachusetts Land Court cases they filed in 2022 and 2023 challenging the MBTA's ownership/Eminent Domain of the ROW since at least 1977.
The MBTA reserves the right to reactivate rail. The lease with Eversource does not prohibit that, at least according to the STB.
The diamond is supposed to be placed inside the rail rotary after the Mass Central Rail Trail is finished.
Also, Table 2 from the historic preservation plan linked in the article lists a few dozen items to be avoided or removed/reset, including the diamond junction, Section Tool House, South Sudbury Station, Telegraph Poles, Mileposts, Whistle posts, etc.
https://sudbury.ma.us/wp-content/upload ... -HPAPP.pdf