They're very easy on boxes, When you do the 90 on, try and overspin, but only a very very small amount, almost unnoticeable, when you get them on boxes, they'll get really easy, and then rails are basically about comitment, if you don't commit, you won't get it. I find it easiest to keep looking around right after you land on to railslide, and jump almost immediatley.. that way is easier on short rails.
first obviously be able to do bs 270s out cuz similar to a fs sw up u just throw it earlier but learning them on a-frame boxes is really easy
but for a down rail i think you get good grip so ull have more control then a box plus it looks way cooler on a down rail but roll up aggressive be ready and commited and throw it early to learn it like as soon as you pop on use you forward hand to guide you around and jump. u gotta be commited cuz if you puss then ull just come off the rail and only get the tips on when your coming around unnatural so man up pop and set it down underfoot