Put the summary of the bug you're having here

I expect to create a minecart, jump in it and for it to run smoothly and for the environment to move, not the mine-cart

What happens is the environment moves but the cart shakes as if I am in two mine-carts even though it is definitely one

Create a minecart. Push it. Jump in. WHen minecart is moving, mine cart is 'shaking'