By Krueger - https://www.indiedb.com/games/the-long-dark/images/island-map-great-bear-fan-made
It really lays bare some missing pieces. Like there's no road that goes to Mountain Town currently that connects into the wider network. And the future rail link after it terminates in Coastal Highway certainly has to go north to either Port Mary or Perseverance Mills.
Mountain Town is located "about halfway between Port Mary and the interior mountains", so it also may connect there, even if it's blocked off with the cave-in. So it makes sense for MT to connect via Blackrock to Port Mary. It's hard to see how that'll work - maybe MT to Pleasant Valley or Keeper's Pass. Then that would plausibly connect it to BR and PM.
Edit: Keeper's Pass South has a collapsed tunnel that definitely leads to MT, so that answers that. So the Port Mary connection probably happens via Pleasant Valley, past Thompson's Crossing through the blocked-off curve there.