On rainy days there is a strong urin smell on trains in Tokyo. I suspect this smell comes from ammonia. On Wikipedia I read:
Ammonia and ammonium salts are also found in small quantities in rainwater […] Ammonium salts are found distributed through fertile soil and in seawater.
Both is valid for Tokyo. In rainy season especially there is a lot of rain. And Tokyo is located at the sea.
Air conditioners work by drying the air. Tokyo trains make extensive use of air conditioning. Could this process intensify or make the ammonia to fall out and thus create the strong urine smell? What other possibility is there for the strong smell?