You need to define "friendly to waders".
I fished the Yakima for the first time in the 90's – I was my very FIRST west coast stream that I ever fished. I met up with a buddy from West Seattle to took me to see the amazing basalt canyons of central Washington, herd of mule deer running thru the valleys and uplands, and of course to spend several nights at Red's Campground.
We parked along the canyon and hiked down to a couple of good spots to wade during lower to normal flows. It was late summer/early September during a drought year, so that helped. Lots of hoppers and ants.
Anyway, the wading was tougher trying to straddle rocks here and there, but it was manageable.
Of course, I also said something to him that he NEVER lets me forget… all my fishing was eastern rivers with smaller gravel/rocks that are worn down over time. The wading tends to be easier to walk around in. Of course, in my haste I asked him "All all the rivers out here the same?" He asked what I meant, and my reply was "You know… with all these big rocks in 'em."
He laughed maniacally and then managed to explain, which I already knew, but it sounded so stupid that he never lets me forget that.
If you're out on the Yak, check out mile posts 18-22 – there are a bunch of rock gardens and the fishing can be good. You can pretty much wade anywhere along the Yak from the road.
I think you can access the railroad along the other side by MP 9 near the Slab campground.
Have fun.
R