I've rode a mine cart many times. I've only made one mine cart in the game (sensibly cause I have one rail system). I was headed to the second building that I created when I found out that I was forgetting things that I needed to work on it some more. I dismounted the cart, and destroyed it, and placed it back at the beginning. I grabbed the items I needed and rode my mine cart back to the other building. Come to find out that there was another mine cart sitting at the other end of the tracks. To confirm that it wasn't just a little bug that happened once and never showed itself again, I did it again and ended up with a total of (yes) 3 minecarts. 1 on the rail and 2 in my inventory. Don't think this is supposed to happen.