What I expected to happen was...:

I thought that droppers would pull items from a minecart with a hopper.

What actually happened was...:
They don't pull the items

Steps to Reproduce:
1. Place a dropper.
2. Place any type of rail above the dropper.
3. Place the minecart with the hopper on the rail.
4. Put items inside of the hopper.
5. The items doesn't get sucked in the dropper.

*Same effect without rail