There are 2 parts at Kinzigtalbahn that have tracks, one ven has signs and level crossings that player can't go to. My theory is that Dovetail were making those parts but they realised it's better for them to make it as separate DLC. More on that I found a video.
These parts of the route are only there for either AI services to function better or because it had to be canceled do to limited budget or the sake of performance. Kinzigtal itself is one of the biggest route in case of performance usage, which is why they cannot even layer the 430 on it at the moment. They wont bring those parts as extension dlcs for it either. Only chance for the northern main route to be playable at one point would be if DTG makes the route Frankfurt to Würzburg. That probably wont be a thing within the next 1 to 2 years as they made it official to leave the Frankfurt area for the time until big steps in performance optimization have been made.
