The bug

If a rail is placed above a top half trapdoor and then the trapdoor is opened, the rail does not break until it receives a block update. This is not the case for ladders against open trapdoors when that trapdoor closes.

Credits

Special thanks to tryashtar in this video for discovering that rails can be placed on top half trapdoors.

How to reproduce

1. Place a top half trapdoor and then a rail above it.
2. Open the trapdoor below the rail.
   → The rail does not break.
3. Update a block near the rail.
   → The rail breaks.