On Sherman Hill, come off of I-80 at the exit for Ames Monument (Vedawoo?). Go past the monument until you hit the tracks. Cross the tracks and turn right. Before you hit the ranch gate, look for a little spur road on the left. I parked there and just threw a sleeping bag on the road in front of my car (no place to put a tent, though). I got serenaded by nearby coyotes, and in the morning I saw one go running by maybe 100 yards away. This spot is outside the RR right-of-way, and is unfenced by anybody else. No "keep out" signs. It still might be private property, though.

Leroy, Wyoming (between Evanston and Green River): Take the exit east of the tracks. Follow the (dirt) road until the trees just before the railroad crossing. These trees are outside the ROW fence, but may still be RR property - I believe that they were for shade for the housing of a section gang. Surrounding land is probably BLM or a rancher's, but there are no "keep out" signs.

Frost, California: There is a California state park (?) campground right next to the RR just below the Frost flyover. The campsites next to the tracks are less popular due to train noise, so even if the campground is busy, you can often find a spot there. This one is a full, formal, legitimate, with-fee campground.