If you have a mine-cart track system in your world and yo have the problem, "do I go left or right." After many failed attempts I have figured out how to build one;
You need dirt or grass
stone or stone brick
Step 1: setting up
OK, as the first picture shows you have a track going left and right. to start dig out 3 blocks( see 1st and 2nd pics)
place 3 powered tracks in the 1 by 3 hole you dug. make it look like the 1st pic.