there are portions of the right-of-ways that are definitely privately owned, but not near where the trails i found are. i grew up along a powerlines right of way and my parents used to get very annoyed with the behavior of some of the snowmobilers who used it. so, i have a healthy respect for landowners who have powerlines crossing their property and would never trespass on that. but if they're owned by the power company, and the trails are open, i might have to check out where they go...
i know there's one right of way near me that's always used for 4-wheeling. it's right next to a highway and you can see people in there all of the time. i think if the power company wanted to keep people out they would have by now, or if it were privately owned. but you never know...