Introducing..... Shibayama Railway Line. It has 2 stops and its length is 2.2km.
Luckily, all train are through service to the Keisei line, so we can access the line from the Keisei Narita station.
Higashi-Narita (East Narita) station
The first station is Higashi-Narita station, which originally served as a terminal station of Narita Airport.
You can see it was named Narita Airport station, before the opening of Narita Airport Rapid Railway (type three railway company), which provides new terminals for Narita Airport. Now, it only serves as a station to access nearby infrastructure (Airport Police Station, Cargo Area...etc)
You may still access the station via Narita Airport Terminal 2 station.
Spooky lobby
Ground entrance
Skyliner!
Platform...
Unused platform, which still has a Narita Airport station sign!
Shibayama-Chiyoda Station
A one-sided platform which provides access to JAL, ANA maintaining facilities. It is opened in 2002. Nothing really special about it.....
As you can guess, the line is just an extension to the old (keisei) Narita Airport line. Due to some history, it is not operated by Keisei, but as an independent company (which partly owned by Keisei). For this reason, the line remains as a "shortest independent railway line".
Moreover, they also have their own (and only) train which is leased from Keisei and repainted to green livery. Still, most of the time, you only see Keisei train running on it via through service.
Special note: The line between Higashi-Narita station and Keisei Narita station is called Keisei Higashi-Narita Line, which is 7.1 km long.
In case someone is still confused, here is the route map of the line :)
or a better version