Over four miles of retaining wall and two miles of noise barriers will be part of the Green Line Extension project, according to a recent MassDOT presentation.
And, while the new trains will bring noise, planners believe the new walls will leave abutting neighborhoods quieter than they are now.
"Future noise levels are expected to be lower than existing noise levels with the noise barriers at most locations along the existing commuter lines," one presentation slide said.
In all, about 4.5 miles of retaining wall and 2.4 miles of noise barrier will be implemented along the Green Line Extension's route. The project will extend the line through Somerville mostly along an above-ground rail bed currently used for commuter trains to a terminal station at College Avenue in Medford, with seven stops along the way.
About one mile along the tracks will have both a retaining wall and noise barrier, according to the presentation, made in late May.
Prior to the noise barriers, extending the line will add about 10 decibles to noise conditions in areas where there is not already commuter rail tracks, and a few decibles to areas where there are commuter rail tracks, according to the presentation.
The barriers, which will vary in height from 6' to 23', will aim to reduce noise five decibels or more and eliminate any potential severe noise impacts.
In all, 133 residences near the rail bed will have a noise barrier or retaining wall, and 37 residences will have a view over one of the walls, according to the presentation.
The Green Line Extension project is being implemented by the state as part of a settlement related to air quality mitigation stemming from Boston's Big Dig highway project. The extension is expected to be completed by 2020, and is projected to cost $1 billion.