Thanks to Jane's report, I decided to try the Salmonberry Canyon.
I started from the West end where Jane did.
I went about 5 miles before I turned around. I think the other end is about 6 miles.
At many places the river has under-cut the railroad:
But there's a fairly easy way around staying on firm ground:
A couple places there is debris over the railroad but there is a route through it that's not too bad:
At about mile 5, after the first tunnel, there was a creek that washed away the bridge out from under the railroad. I saw no way across the creek, without getting my feet wet so I just walked on the railroad ties, that were hanging from the railroad tracks that were hanging from both sides of the creek. At the end, one of the ties fell off so you have a large step. This made me somewhat nervous but it seemed solid. There was much more weight of logs and rocks than what I weigh. You can see the wooden bridge down below in the creek bed with the fiber optic cables next to it:
Then, after the second tunnel, the railroad track is now located at river level. There was a way to walk through there, but I had walked about as far as I wanted to so I turned around. I talked to a couple guys who had walked all the way from the other end:
I camped overnight next to the track, about at mile 3, near where the track goes from the North side to the South side, right next to this railroad construction machine. There were also places closer to the river, further away from the tracks:
One day I met a couple guys who had walked from the other end. The other day I met two Fish and Game people. At the trailhead a saw a few fishermen. This was all mid-week in February. I saw foot tracks of quite a few boots so you might well see people here on the weekend.
There is a lot of damage to the railroad. I could see it never being reapired. There were about 20 places where the river undercut the railroad track.
I talked to a couple fishermen that said the Nehalem and Salmonberry Rivers are both prime Steelhead habitat, so expect to see fishermen, and if you're into fishing, this might be a great place to go.
I stayed overnight at Henry Rierson Spruce Run Campground. $10 per night, toilet, picnic tables, fire pits. There is a fair amount of traffic on the road - logging, fishermen?, maybe there are some residences. | 
