Greetings

I Found Mineshaft, but there was a problem, zombies and creepers couldn't be able to move on Rails! and I'm sure It's similar for the other groundtype mobs.
It looks Cool and useful but I think, it's not normal and Also It's unusual for such this Low-lying block (rails).   

I've just found the bug.

It depends on you to keep it or fix it.

With respect and best wishes for being better