Steps to reproduce:

1. Build the following setup:
   
2. Break the rail
3. Place a gravity block next to the stone bricks/under the piston
4. Activate the piston

Observed result:
The gravity block pushes the minecart into the block below it, making it phase through that layer if it is only 1 block thick.

Expected result:
The gravity block lands in the minecart without pushing it below.