I tested this out on all kinds of tracks (inclined, turning, and straight) with all the rail types, and it seems that when a player is in a minecart that is not already moving, it is impossible to move it sideways or backwards. The only way to get it to move by itself is the player is facing straight along the track and holding the "Forward" Key. It's even more annoying when going downhill, as there is already a bug that prevents the player from automatically moving downhill.