which is owned by the third-sector Narita Airport Rapid Railway. All trains on this railway, however, are operated by either JR East or Keisei Electric Railway, with each operator getting dedicated usage of one of the two tracks into the Airport. In this situation, the JR East single-track section is counted in the JR total, while the Keisei single-track section is counted in the Keisei total. Ridership Both average daily and annual ridership are included, because only average daily ridership or annual ridership (not both) is available for some operators. In cases where data for only one of the two is available, care has been taken to not extrapolate the passenger volume to obtain the other, as there is a potential margin of error when attempting to derive average daily ridership from annual ridership (which is usually rounded to the nearest thousand passengers) and natural disasters or other unforeseen situations may force some operators to shut down for extended periods of time, as happened with the Sendai Subway in the days following the 2011 Tōhoku earthquake and tsunami. Categories For readability and ease of comparison across metropolitan areas, systems within each metropolitan area are broken down into the following categories: Subways are divided into two types: publicly operated and privately owned, and are grouped together with each other regardless of ownership. See for more details. : Systems generally considered "subways" and operated directly by government agencies at the city (e.g., Kobe Municipal Subway) or prefecture (e.g., Toei Subway) level. : Systems generally considered "subways" that are owned by private operators (e.g., Tokyo Metro), as well as third-sector (semi-public) subways (e.g., Minatomirai Line). : Any of the 15 private railways (excluding subways) considered by the Ministry of Land, Infrastructure, Transport and Tourism and others to be the largest private railways in Japan (by network length, ridership volume, and other metrics), providing critical urban rail service in the Greater Tokyo, Greater Nagoya, Osaka-Kobe-Kyoto, and Fukuoka-Kitakyūshū areas. Japan Railways Group operators such as JR East or West Japan Railway Company (JR West) are generally not considered major private railways because they are descended from the government-owned Japanese National Railways (JNR), despite their size and their status now as private, for-profit railways, following the dissolution of JNR in 1984. : Any of the eight private railways considered by the Ministry of Land, Infrastructure, Transport and Tourism and others to be intermediate in size, smaller