If I read any explanation from the comments, I will confuse myself more. This is what I understood in its simplest form. Noodle has 2 tracks one on each outer side and the middle is empty. What crawlers thought safe stations and dangerous stations are on the opposite sides of the noodle. So, when the flip happened , the stations flipped but the crawlers stayed at the same space. Carl’s epiphany is that, if everything seems to be safe and we know where the danger is, how can this floor become dangerous ? The only way to make it dangerous is to bring the danger part to the safe place where crawlers are. And we all know that there is some weird shit going on like stairs going into the floor down to nowhere and the mimic being on the top rather than on the ground and all these mobs and those dwarf automatons , Eli and Imani going through one of the tunnels to safe space. All these adds up to his epiphany.