When sending a hopper minecart over a powered activator rail it becomes and stays locked (as intended according to Dinnerbones comment on MC-7663).
But unloading and reloading the minecart resets this. Simply adding an "isLocked" NBT-Tag to it should fix this issue.

What I expected to happen was...:
Unloading and reloading shouldn't change the behaviour.

What actually happened was...:
A locked hopper minecart picked up items after reloading it.

Steps to Reproduce:
1. Push a hopper minecart over an powered activator rail.
2. Push it off the rail.
3. Throw some items at it (it will not pick them up)
4. Reload the world.
5. Throw some more items at it (it will pick them up)

Video: https://youtu.be/ZkI3uU7hoBA?t=2m13s