usage=main is not synonymous with "main line rail" either. On OpenStreetMap, usage=* is indeed subjective. Official government classification is not the only factor we consider when determining the value of usage=*. As with roads, we prioritize connectivity to the national network when determining how to classify them.

We don't retag things to make them show up a certain way on a particular renderer; see [1].

I've gone ahead and reverted the South Shore Line back to usage=branch [2]. To avoid disputes like these in the future, I strongly encourage you to discuss any large-scale edits on the community forum [3] or the OSM-US Slack [4] before proceeding.

[1] https://wiki.openstreetmap.org/wiki/Tagging_for_the_renderer

[2] https://www.openstreetmap.org/changeset/156450223

[3] https://community.openstreetmap.org/c/communities/us/78

[4] https://slack.openstreetmap.us/