i found a serious problem with carts. When someone is riding, you can see his copy above the rails in place where he start driving.
That copy dissapear, after he stops riding.

reproduction:

1.two players are standing close to the rails
2.first one should put own cart on rails, and then get "inside"
3.second player is standing on ground and watiching.
4.After frist player will start to drive away - you will abe to see COPY of him hovering above the ground.
5. Even if first player will gone - i mean drive behind your screen, you will able to see his player copy as well. But always in pleace, where he START to drive in cart.

CHECK screenshot number FIVE and SIX.

and by the way - check his leg, when he is inside cart - his leg are outside of cart

cheers!