Railroading is a common term among stitchers these days on message boards. Basically it means separating the threads as you stitch so that they lay flat and create a much smoother neater appearance for your finished piece overall. Many new stitchers think this is a time consuming and unneeded process. The truth is that its not time consuming at all and in the long run will save you time and help you create a beautiful piece in the end.
There are two ways to do this, which you use is a personal preference on your part. The first way is to railroad both the bottom leg / and the top leg \ of your cross stitch. The second is to railroad just the TOP \ leg of your cross stitch. Either will give you a neat appearance, personally I railroad both legs of my stitch. You also want to do this with your backstitching.
Now, for the directions.
Bring the needle up through the fabric like you normally do, but before you go back down to create the leg of the stitch you place the needle between the two strands of floss. So it will be thread / needle / thread then with the needle between the threads you go back down through your fabric.
Doing this with your cross stitches is going to create a smooth appearance in the finished product and its also going to prevent those nasty tangles we all hate to pick out of our floss! With less twists and tangles in your floss your going save yourself time and floss because you get more flat stitches out of a length of floss than a twisted and tangled length of floss.
I hope this helps you in creating your next beautiful masterpiece and as always Happy Stitching!