What I expected to happen was...:
Let the animals walk through the rails as easily as before.

What actually happened was...:
The rails block the movement of the animals like a high wall.

Steps to Reproduce:
1. Place a circle of rails around your dog.
2. Try to go far.
3. Now, no matter how loyal your dog is, it won't follow you anymore.
4. Because the "railway high wall" trapped it.
5. Other mobs will have the same problems (zombies, villagers, cats, and maybe horses and donkeys)
6. If the player pushes the dog onto the track, the dog will follow the owner again. But the "railway high wall" is still there.