Norms and mutants
Tokyo’s underground is very extensive. Subway stations are generally very clean, well maintained, are outfitted with modern equipment designed to enable and streamline the immense commuter flow, and at major commuter hubs the underground concourses spread and connect with each other such that, if you are lucky, on a rainy day, or during a typhoon or snow storm a person can spend most of the day under a roof, without having to go outside into the elements.
Often when I am commuting I play an imaginary game with myself. (More than one, actually. If I am not reading or sleeping on the trains and subways then I am usually fantasizing stuff.) While I’m walking through stations and seeing how they are built and connected, how deep they go, how many basement levels, how many pillars they possess to support the streets and buildings above, how long the connecting underground corridors are and how they are built, how many exits there are and where they are situated I imagine American soldiers fighting Japanese in the city in 1945 one neighborhood at a time - at a very great cost in human lives, of course - even one subway station at a time. How would they advance? How would they take advantage of the pillars and other obstacles? Where would they take cover and how would they advance across open spaces? How would they safely and effectively gain entrance through stairwells and penetrate the tunnels? How could defenders stall them? Where are the best ‘kill zones’? Where are good places to set traps, and what kind of traps would they be? What would the price be in dead and wounded to advance from here to there under fire?
Then I renovate the game into a “Beneath for the Planet of the Apes (1970)” kind of thing (except without the apes) and imagine a post-apocalypse world in which no human life remains on the surface, and all the survivors are taking refuge and living in old urban subway tunnels. I imagine ‘normal’ humans, or Norms living in the deeper tunnels safe from radiation, and radiation-damaged Mutants occupying the tunnels and spaces closer to the surface. Everyone lives entirely underground, and every night - it has to be at night - they battle each other (like the 1945 advancing American soldiers scenario) with antique and rudimentary weapons for control of the tunnels and stations, to expand their territories until, finally, there are little kingdoms of warlord-led Norms and Mutant in the old subway system, each trying to exterminate the other. I don’t know why they should want to exterminate each other. That’s not part of story.
I have an active imagination.