Hostile Mobs spawning on minecart rails in two block high tunnels

Converted server from 1.7.10 to 1.8. Existing minecart tunnels with rails and 2 blocks high are now allowing hostile mobs to spawn. Have seen skeletons and creepers so far.

The specific tunnels here have glass for the top and top half of sides and are under water. Not sure if that is the difference. Will update with further info as itis found.