Not sure if it's post-war but the setting was people have moved and live underground. There are certain people that travel above ground fighting monsters and will fight the monsters underground. They are treated as killers or people that the you don't want to get involved with because they carry out executions for those that have violated underground law.

MC is looking for someone that killed his brother? and the last time I read it their childhood friend showed up that had a crush on the brother and also fights against the monsters. IIRC the fight took place on old subway/train cars that were being used to travel.

Is it Ayanashi?

Yes, that's it. Thank you.