1.) You'll need to get, a sticky piston, 2 rails, 6 redstone, 4 redstone torches, and 2 blocks of your choice (You might also wanna get a map, to make sure that your sticky piston will be facing either, East or the West.)
2.) Get out the 2 blocks of your choice, place them down 1 block away from either. Meaning, they are not side by side, or connecting to each other.
3.) Place the 4 redstone torches on each block, on two sides of the blocks.
4.) Place the Redstone down, in-between the 2 blocks, connecting them up with the 4 redstone torches. Then, place them ontop of the blocks. Now put the last piece of redstone connecting outward, with the other pieces of redstone. (I will show you what I mean by this in the video below.)
5.)Now, get out your piston, and connect it up with the Redstone. Place it down, and make sure it is facing either, East, or West.
6.) And finally, place the 2 rails in-front of the sticky piston.
7.) And there you go, you have finally built your Rail Duplicating Machine.

YouTube Video: