i used to have this same problem i could never get onto rails and stay on them, i'd either slip out of slip off right away. what helped for me was really thinking about how high the rail was from the lip and thinking about how much i actually needed to jump to get onto it. Whenever i slipped off I was literally jumping 6 to 8 inches above the rail and landing on it really hard which would throw off my balance. If i focus on jumping just enough and getting onto the rail smoothly (not landing on it really hard) it lock in nicely and can slide the whole thing.
idk it worked for me.