http://prntscr.com/gja2vo <---- Picture
Creepers cannot walk over rails. This could be used for exploting the mobs and farming them, but it's kind of stupid. Why can't they walk over it?
I'm not sure if zombies or skeletons also cant, but creepers for certain, cannot.
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Thank you for your report!
However, this issue is Working as Intended.
As per MC-33359, mobs avoid pathfinding through rails to prevent stopping minecarts; this is an intentional behavior.
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