With birail 3+, Select your profile curves first (the vertical ones) and hit enter. Then, select your rail curves (the two horizontal ones) and hit enter again.

Just a little tip: You can look at maya's help line at the bottom of the screen and it will tell you what you need to do as far as selecting (select your profile curves and hit enter)...after you do that, it will then tell you to choose your rail curves. Make sure when you select the curves, you select them in order, same as if you were doing a loft. In other words, select them right to left or left to right -- don't jump around. Hope this helps.