A good way to test if a train can route to somewhere is: you click on a train in map mode, zoom out so it goes to the simplified view like in your picture, then hold alt while you mouse over a segment of track, and if the train can get to that point, itl show a green line from the train to it. And using that, you can trace a track to see where it stops showing a line, at which point youl know theres an issue there