What I expected to happen was...:
The rails is pushed down.

What actually happened was...:
The rails is pushed down and it drops as an item. (So you have still the rails and a new item)

Steps to Reproduce:
1. Place a solid block with air below it.
2. Place any kind of rail on top.
3. On top of that place an downwards facing piston.
4. Activate the piston.
(See screenshots...)