When a mob is chasing the player, or just randomly walking around, the mob will avoid walking across the rails.

How to reproduce:

• Make sure you are in Survival mode
• Place some rails
• Go on one side of the rails
• Place a Zombie on the opposite side
• Watch as the Zombie will walk around the rails when chasing you.

I have also recorded a video of how it looks and how to reproduce it, however, the file size of the video is too big (around 83MB), so I had to upload it on YouTube: https://youtu.be/TeiyNBH7B4I