We regularly ride and explore powerline trails all the time. If it's not marked no trespassing and there aren't any gates I'll go for it. There are a few areas around here where the powerlines cross through hunting leases though - so be mindful of that - especially during hunting season.
Occasionally I've come out the back side of someplace where I see no trespassing signs marking where I just came out of (but there weren't any where I entered). I haven't had any problems but obviously respect the signs when I do see them or know they are there.