Minecart falls in block when going down

Put the summary of the bug you're having here

What I expected to happen was...:
to test a hopper minecart

What actually happened was...:
when the mine cart went down it like fell in the block

Steps to Reproduce:
1.make a little rollercoster that goes up and down
2. put a minecart on a rail and push it
3. watch the cart.