There are a couple camping pads right along the Iron Horse Trail. Tent pads, toilets - very lux for the backpacker, actually. Not sure exactly where, but I'll bet Ranger Google would know. I know there's at least one between the Pass and Rattlesnake.
What's your total route? Easton to Rattlesnake? Just curious. I live right by Rattlesnake and have been toying with a trip along the Iron Horse for awhile. I've dayhiked chunks of it, but mostly to get to other locations that shoot off of it. I'd love to do a Columbia River-to-home trip. Kind of want to walk it, actually, but I also think it's time to start playing around more with bikepacking so might got that route first.
Definitely post up a quick report! I'd love to hear your experience. Not a typical backpacking trail for sure, but lots of possibilities for longer distances and shoulder-winter season walks or bikes for sure.