Not so much a train of thought as a meditative habit- I'll close my eyes in the shower and imagine that I'm actually outdoors in a thunderstorm. Not all that fun when it actually happens, but something about the idea of thunderstorms while you're in a warm, indoor environment is very cozy. Totally recommend this for other anxious types like me x')
As for actual shower thought tangents- "what if a hero's superpower was spontaneously generating pizza? Would that mean they were summoning the pizza from a pizza dimension, so for every pizza, something in their world would have to be displaced? Could the pizza have any toppings at all? So like, uranium pizza? Shotgun shell pizza? Would that make them the person responsible for supplying their superhero group with everything from snacks to ammo? Maybe they would just ironically be called "delivery man" and even wear the uniform...maybe there would be a subplot where all the heroes have to go to the pizza dimension to fight the invasion of the sandwich people...wait, does that mean the pizza is sentient?"
I neither read nor write much in the way of superhero stories, so I'm not sure what brought this on. Probably just craving pizza. >_>