Minecarts cannot traverse corners

Place down a line of normal rails then one of the ends go off on a 90 degree turn.

Place a minecart and try to make it go around the corner. Whether you push the cart from outside, use powered rails, or ride in it and hold W, it is impossible. It does not work for rails placed in a 2x2 circle or with other types of minecarts. If you are pushing it from outside, it may drive off the rails entirely on a corner.