The agent from whom you purchase the Rail Pass will probably send you one with the exchange order. Even if they don't it only has 3 or 4 lines and can be filled out in about a minute.
I don't know if this is still current or is printable. The last time I took one with me they told me it had changed and I had the old one. I filled out a new one they gave me very quickly while the agent was looking at my passport.
http://s874.photobucket.com/user/jpl...s/App.png.html