Mob Crossing
This *server* mod allows monsters to cross rail. Due to pathing issues, by default mobs are unable to walk across rail unless they are forced on to it. This allows a line of rail on the ground to block mobs from entering an area. This issue has been reported as a bug by minecraft/mojang users many times but they always mark it a duplicate.
One example of the bug reports is
MC-33359 : Mobs are unable to cross minecart rails
This mod will allow modpack designers and server operators make the game more realistic and more difficult.