"Where did this info come from?"
From the maps of KWG & Cliffs where their RR and N/S roads were to go in over the eskers. That path misses Marten Falls by over 40 Km. The winter road to Marten Falls could share that route only part of the way. Then what?
I do not know the region but rely on feasibility studies of previous years and the N/S esker route is one possibility and the other is the E/W. If there were other then I gather that they would have been mentioned.

Yes the KWG alternate route is a composite ( compromise?) of the previous two.