怎么判断第四层是老去的／limbo层是年轻的： 因为年轻男女主卧轨死后紧接着在现实中醒来，说明年轻男女主在limbo中——那么老去的男女主卧轨必然发生在第四层； 另还有一个细节，就是年轻男主在卧轨时说的台词，最后一句让女主接，而女主接上了——这说明他们之前说过这段话（如同后来男主和老去的日本人就是靠互相接话来确定自己是在limbo中一样），说明他们之前卧轨过／另“You know where you hope this train will take you, but you can't know for sure.”也意味着他们之前用过卧轨的方式想回到现实，但失败了。
附上经典台词： You're waiting for a train, a train that will take you far away. You know where you hope this train will take you, but you can't know for sure. Yet it doesn't matter.Now,tell me why!- Because you'll be together!
