What I expected to happen was...:
The piston would break the rail and push you along.

What actually happened was...:
There were two rails facing downward with no block to connect to.

Steps to Reproduce:
1. You place a piston one block above the ground and put a button next to it.
2. You place a block in front of the block the piston is on and put a rail on it.
3. You put two rails one block below the first rail and press the button.