mobs are spawning on top of half slabs and rails

This is working as intended.

Mobs can also spawn on the tops of upside-down slabs and double slabs, but not on lower slabs.

http://minecraft.gamepedia.com/Slab

Thank you for your report!
However, this issue has been closed as a Duplicate of MCPE-14467.

It has been linked to this report. If you have additional information, please add it to that report.
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