In different countries, the conditions and thresholds for taking the subway are entirely different. This is also a privilege granted to different countries by the Underground Federation.
Because of the constant temperature barrier, the Winter Principality can only use the subway for inter-city transportation during the winter years... Additionally, with the omnipresent Hands of Winter responsible for surveillance, counter-espionage, and assassination work, it actually makes their threshold for taking the subway quite low.
Aside from a few targets under surveillance who are not allowed to enter underground—most civilians can take the subway as long as they have money.
But it's different in the Kingdom of Noah.
In the Kingdom of Noah, more than 90% of the subway capacity is used for transporting goods.
