Replying to Urban rails.

Hey guys, I need some help on how to lock onto urban rails. Now, I'm not talking about stair sets and stuff out on the street, just normal hop on ones that are going down. I get on but more then half the time I slip out, can you just tell me the step by step process you guys do, how far away you are before you jump on, where you look(skis, end of rail). Just let me know, thanks guys! and the rails I'm talking about are like this. I'll put a pic below.