The minecart seems to float after breaking the rail below it.
This is discovered while testing MC-2758 and relates to MC-505 since the boats and minecarts are both entities.
You can move them by pushing them, but it will still be floating. This seems to happen in both Survival and Creative.

Steps to Reproduce:
1. Put a minecart on a rail
2. Mouse-over the rail.
3. Break the rail
4. The minecart floats! :O