My idea is that they are short-cutting through Chicago on a circuit around the forest preserves, and that it's a seasonal trip. Or perhaps there are two or three different families/clans that stagger their trips seasonally. Its size is no smaller than many (the majority?) of BF structures. These would be communicative in function, maybe a map. I'll try to illustrate my thinking:
Suppose you want to get from point A (sightings in the South) to point B (my structure findings at Sidney Yates over the summer):
You're not averse to traveling unused rail lines in the dark of night, so you have two options that could take you NE:
Suppose you took the nearer option (left arrow), now we zoom in to UoC area:
So you come in at the white arrow, have a clear path along the green (structure marked as pink asterisk), the other rail line then takes you up along Lake Shore Drive (with a decent bit of tree coverage between the tracks and the road, this is my drive every day). This could be represented by the structure as:
You ultimately would need to transfer over from the lake shore to the river, which would likely involve the yellow path (I-90) briefly, then the river gets you the rest of the way there:
How confident am I of this theory? Not at all. But this was already an idea I had brewing back in May, and this morning's structure fits nicely.
Yeah, it all seems overblown and crazy, and yet, there it is, again.