Japan operates a variety of through-services (直通運転, chokutsu-unten) or nori-ire (乗り入れ) or through trains which are direct seamless connections between rail operators, using leased trackage rights and junctions, to cut cross metropolitan area commutes without having to change trains, wait, figure out connections, or cross platforms/stations. Most of these junctions have been constructed well after the lines have been operating. Some far-flung spur lines have been shortened (abandoned stations and track due to low patronage); through service is a frequently used method to integrate the surviving short stub lines into multiple rail operator's systems, enhancing convenience and ridership. Through-service proliferation is increasingly common phenomenon as the railway networks, urban density (and Japan's demographics) have matured and new line construction is minimal.
There are several dozen unique through service runs in Japan, the actual track usage details are complex (but as a seamless service offers an abstraction layer for the end user), the Japanese Wikipedia page has a complete list. Standardization is required for cross railway services, in terms of rolling stock dimensions, rail gauge, overhead power, not to mention coordinated scheduling between a mixture of separate companies and/or government agencies.
Through services in Japan are commonly of two types: unidirectional or bidirectional (unidirectional trains return empty on non-circular tracks). The first usage of through services was on the Tobu Kameido Line in 1904, but was suspended in 1910. Japan's first modern (postwar) through services began in 1960, from the Toei Asakusa Line to Keisei Oshiage Line.
There are also through services that have been discontinued (not listed here).
Asakusa Line route