I have always thought that rungs are safer than rails because you can overlap your fingers and thumb on a rung, but not a rail. If you were to slip and only had grasp of the ladder with one hand, which would you rather have a hold of, the rung or the rail? I teach my firefighters to keep their hands on the rungs. If they are carrying a tool I teach them to keep one hand on the rung and the hand carrying the tool to slide behind the rail. Most importantly always have 2 points of contact at all times.