Minecart don't drive down a sloping rail

+If you place a minecart on a down sloping rail, it stays on the rail and doesn't drive down.

What I expected to happen was...:
The minecart shoud drive down the rail until it doesn't go down anymore.

What actually happened was...:
The minecart doesn't move.

Steps to Reproduce:
1. Build a down sloping rail.
2. Place a minecart on the sloping rail and it doesn't move.+