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