I am not entirely sure if this is a bug or a new intended feature, however I am noticing all types of mobs move under their own authority while riding inside a minecart while on a minecart track, even on unpowered rails. I also noticed this behavior specifically with Villagers in 1.9.

"What I expected to happen was..."
The mob can not control it's movement while inside a minecart on some type of minecart rail.

"What actually happened was..."
The mob can control its own movement and the direction of travel while inside a minecart that was on a rail

Steps to Reproduce:
1. Place mob in a minecart that is on a rail.
2. Place bait for mob inside of minecart to move towards.