Rail slides are a trick the derived from skateboarding. Skateboards commonly had rails that protected the bottom of the deck from damage when sliding across it and allowing you to slide faster and further than you could normally. To know what a rail slide is you must think of a skateboarder sliding down a handrail along some stairs.
While you may never find handrails on a normal snowboarding run, the principal of a rail slide is just the same and the application is very similar. Instead of sliding down a handrail you may wish to slide along the lip of a ramp or the edge of a ridge of snow. You are only limited by your imagination as to where you can use a rail slide once you know how to.
There are several elements to a rail slide, all of which must be mastered to some degree before your can move on to a rail slide. You must be able to Ollie in order to land on top of the edge that you wish to rail slide along. You must be able to twist your body 90 degrees in the air so that you land perfectly centered on your rails. You must also be able to center your weight on your snowboard and be able to balance not matter how the surface changes on you while sliding down it. And then there is the landing. To land you must either Ollie off the edge or are able to twist 90 degrees again in order to be facing straight ahead afterwards.
Now comes the fun part, putting all the elements together to rail slide. Begin by riding down hill along side the rail that you wish to slide on and a reasonable rate of speed. When you are ready start to cut in towards it and Ollie so that you are above the rail. Now comes the hard part, landing on the rail. If you timed everything and angled everything well, you will have to do nothing but turn 90 degrees in the air and you will be ready to go.
Since we are not perfect we will assume that there is going to be the odd problem when learning to rail slide. The biggest problem most people face is centering the snowboard on the rail. The only real way to get the hang of this is to practice and bail a few times. However knowing your riding style may decrease the amount of time it takes to get the hang of balancing on your rails. Generally it is easier to balance if your heels are pointing up hill.
Many people find it beneficial to learn to rail slide by dusting the rail a few times before actually riding the rail. To dust the rail you angle your approach so that you are riding at about a 45-degree angle rather than parallel to the rail. Once you begin to get close Ollie up so that you are just barely above the rail. As you glide over the rail lower your feet so that your snowboard touches the rail before landing on the other side of the rail. As you become more confidant with your skill touch down on the rail for longer periods of time. You will want to change the angle of your approach so that you are closer to a 90-degree approach to the rail as you wish to stay on the rail for a longer period of time.
As you can see rail slide are not hard to do once you get the hang of the balance points. Practice balancing on your snowboard as much as you can before trying to rail slide. Many people find it easier to balance at higher speeds so make sure you are proficient at the base skills required so that you can rail slide at any speed. The key to this trick really is practice so get out there and begin trying to figure out your own balance points.