Great find!
You can see how in that western continent, there's only a yellow line. I'm assuming it's the train (especially since that seems to be the continent Niflheim is (look at the airship, for example) in and because of the snowy mountains), so the whole "second half or so of the game is more linear, but you can return to the previous zones". Also, Tenebrae is closer to Duscae than I thought it would. Makes sense if Noctis and Regis travelled there for a while.
I hope we can still explore freely even if we don't have the car or something like that. I wonder if we can travel to some of those islands with the aircar, considering what Tabata said about otherwise unreachable places. Considering we see the Malboro place in that continent, it seems that way (it'd be weird if they heavily restricted our moves).
Can't wait.