I have a touch of rail phobia, a mild fear that I will collide with the kick rail in an indoor arena, and I keep wondering if anyone else has it as well. If I ask others about it, they look at me like I'm nuts. The issue for me is just at the walk. I'm okay on the rail at the trot and canter, although in a new arena I do keep an eagle out for anything that might protrude, like an eye hook.
My phobia dates back a few decades when I heard of two instances of riders dying while riding alone in an indoor arena. The first appeared to happen when the horse apparently had a heart attack and crushed the rider between his body and the kick rail. In the second case, the horse seemed fine but the rider was found dead shoved up against the ring wall. I don't recall if she had specific injuries.
I always fret a bit at the walk because if the horse stumbles and the riders falls off, there is no momentum to project the rider away from the mass of the horse, as there is in the trot or canter, so in that case she'll connect with the wall pretty hard. When I'm riding on a loose rein at the walk, I'm always well inside the track. Of course, it's just good manners to walk off the track when sharing the ring with other riders, but habit has me also doing so when I'm by myself.
The practice may have saved me once when my mare fell sideways and dropped to her knees as I asked for the first trot that day. I was able to get us both upright and then jumped off...turned out she had an abscess on a heel that was getting ready to burst. We both ended up just fine. However, if we'd been right next to the rail we would have slammed into it and the outcome likely would, at best, have been some bruises.