To answer my own question: those are mined paths, as one of the posters above surmised. So says the designer in a post at Consimworld:
In the draw near the middle of the map, there's a dashed red-brown line that passes through the middle of hexes
The dashed red line in hexes 0816, 0817. 0917 and 1017 is a mined trail -- cannot be entered by US armor and artillery until cleared by an engineer base in the extended game.