Minecart swimming position bug

Put the summary of the bug you're having here

What I expected to happen was...:
normally working minecarft with Steve in it

What actually happened was...:
i "jump" in to the minecart with position of Steve swimming, i could very hardly move with the minecarft, and i also almost bug to the block level of the rail

Steps to Reproduce:
1. put a minecarft on a rail
2. press right button to enter the minecart
3. BUG