Just type up a piece of paper that says something along the lines of:
"I, _________, hereby swear that I will take no legal action against [your name] with regards to [said product]. Skiing or snowboarding on rails and boxes is dangerous, and I am aware that I am subject to injury, both from my own stupidity, and from the possibility of product failure, and regardless I am completely responsible for any and all bodily, property, and all other damages that may occur.
Signed: ______________________"
Done. If it has his signature on it, it is a legally binding document. It can be written on Kleenex, or even on the rail itself. (not recommended). If he signed, it is valid in court.
At least, such is my understanding. Lawyers of NS, feel free to refute me. (LOL, NS doesn't have lawyers!)