What I expected to happen was...:
That the mob would get in the minecart.

What actually happened was...:
The mob didn't get in the minecart.

Steps to Reproduce:
1. Create 2 seperate railways 1 made of regular rails, and 1 made of powered rails in a fully enclosed space.
2. Put any mob on both railways.
3. Push a minecart towards both mobs and you will see that on the regular rails, the mob will get in the minecart, but won't get in on the powered rails.