Minecraft wiki entry for Trapdoor_uses, associated text: Similarly to making entrances underground you can use trapdoors to drop your minecart onto track underneath making an automatic entrance (good for keeping hostile mobs out, making it convenient getting into your base). It can be powered by the rail detector so the trapdoors will only be open for a short period of time. A minecart will not fall through a single trapdoor (a 2x3 trapdoor setup works fine).