Keio Corporation is one of the major private non-JR railway companies in Japan, operating in Tokyo metropolis and Kanagawa prefecture. The name Keio comes from Tokyo and Hachioji, second largest city in Tokyo metropolis.
Keio Corporation operates 7 lines, including 2 major lines; Keio line and Inokashira line. The other 5 lines are feeder lines of Keio line, including Sagamihara line which is 22.6 km long and Keio New line which is running under national highway route 20 till Sasazuka station and connecting with Toei Subway Shinjuku line at Shinjuku station. Keio line and Keio New line are using separate platforms and these two lines are not transferable at Shinjuku without passing the ticket gates.
Keio Corporation is one of the few railway/tram operators in the world which still use Scotch gauge; 4 feet 6 inches(1372 mm) track gauge. Although Keio line and its feeder lines are 1372 mm gauge, Inokashira line is 3 feet 6 inches(1067 mm) track gauge which is common in Japan. Keio line was supposed to connect with Tokyo Toden; tram lines operated by Tokyo metropolitan bureau of transportation, another tram operator which still use Scotch gauge but only operates 1 line; Tokyo Sakura Tram in Tokyo. By the way, Toei Subway Shinjuku line was built as Scotch gauge due to mutual operation with Keio line.
Foreigners might use Keio trains when going to Mt.Takao and Tama zoo when visiting these popular tourist destinations in Tokyo.