In my time around Spokane, helpers frequently ran right out of Yardley and often cut off on the fly at Cheney, sometimes they went all the way to Babb, but usually for exceptionally heavy trains. BN was pretty quick to route westbounds over the SP&S via the new alignment after the Expo 74 track relocations (eliminating the need for helpers). I remember when there were a couple of derailments, one right on the Latah Creek bridge, that they ran helpers, they then were usually added at Empire and cut off at Babb or Cheney (or between those points). BN seemed to like using F units as helpers over the mountain passes---those ol' covered wagons were noted for being good luggers and could really dig in at slow speeds---ex-NP units were particularly good (for one reason, they were in much better shape than their ex-GN counterparts). (Crews told me at the time that 4 F-9s could pull more than 4 GP-9s on the CW branch, even though on paper they were the same---one theory that a hoghead told me was that the weight was better distributed on the F-9s.)