What sort of puzzle is that ? It's a little glitch, not something they'll fix. Can't help you there mate... Can't find it near the "unique" N4 somewhere in the "northern part"....
there is no coordinate system in this game and there are no stations nearby no it's not really easy, but with north I mean very very north around where my train is
Very far north but still south of Koblenz You could use the editor and place a few warning signs around that hole! Edit: That's Königsbach, the Pause Map would've told you