That one little hut right in the middile of Rails and Metal- by that fuel point, which used to be high, but is now low.
Not the tiny little square brick building, but the small wooden shed thing right there next to the fuel point. I'll take a screenshot if you need it.
Anyway, when I tried to summon Falls out of it (it was in my LOS, and unoccupied) the building blinked when I clicked on it (The way anything else would blink if you order an ability cast on it) but the Falls walked in form off-map anyway.
This was only one try, maybe I did it wrong... I'll try again a few more times.