In which country are the world's four busiest commuter rail stations located?

Context: Shibuya Station is a railway station in Shibuya, Tokyo, Japan, operated jointly by East Japan Railway Company, Keio Corporation, Tokyu Corporation, and Tokyo Metro. With 2.4 million passengers on an average weekday in 2004, it is the fourth-busiest commuter rail station in Japan and the world handling a large amount of commuter traffic between the city center and suburbs to the south and west.

Answer: Japan