The basic formula is 20# per ton per per cent of grade with curves and rolling resistance added (uphill) or subtracted (downhill). So Saluda at close to a 5% average grade would require around double the braking effort compared to a common heavy grade of 2.2%.

I think Sierra has some 4% out between Standard and Jamestown, but not sure if they are running lumber. Amador Central probably used retainers, maybe Camino Placerville & Lake Tahoe. We never used retainers at Tillamook when I was there, but there are a few times when they doubled the hill and later were running rock for Carban it probably would have been a good idea. I used to see UP set retainers between Smith's Ferry and Banks, but IN&P doesn't have any traffic or trains on the line now.

There was also a system called "automatic retainers". I would guess it probably was just a straight-air over automatic system that dates way back. I know Westside used it. DM&IR and IIRC Santa Fe's York Canyon Coal train use/used automatic retainers. The engines and cars have to be equipped with an additional brake line.