I am not a big rail fan and am limited in my ability to ride them. In general, start with ride on boxes or ones with a very easy gap that does not require the Ollie to get onto. You will have enough to deal with just riding the box without worrying about the Ollie as well. I recommend doing 50/50 at first where you just ride down the rail and get the feeling for maintaining balance. You will soon learn, that all control is done through weight distribution and any directional control is all done through pivot. Lean too much one way or the other and the board will slip right from under you. Steel is muck slicker than the ptex boxes and you will need much better control on steel.
When you are about to loose and go off the rail, do not fight it, try to go with and jump clear of the rail, Fighting it often cause the rider to fall on the rail or box.
As you are doing a 50/50 down a box, keep your shoulders aligned with the rail, and look at the end of the rail or box. Our lower bodies tend to go where we look and where out shoulders are pointed.
When you are ready to try a boardslide (facing downhill) there is a very natural tendency to lean too far back. We do this because we really fear face planting on these things. It is the number one mistake we all make . To this day, I have not mastered a boardslide for this very reason. The freestyle experts use the 'petting the dog" analogy. As you are boardsliding, imagine you are petting a dog in front of you; this tends to keep you leaning correctly over the top of your board.
Try to find an easy, short, low to the ground box about 2 feet wide to try your first boardslides on. What is great about these is if you mess up and do fall forward, you generally always will clear the box and land in snow.
I find it easier to hop onto the box to boardslide that to spin from a 50/50 position into the BS position, The rotation thing is tricky. Come at the box at about a 30 angle and hop off the hump to get onto the feature, and rotate 60 degrees to land on the box in a good boardslide between the bindings.
I am not good at park, but I hope this helps some...
