Transtube

The rail was a transport system on Gallifrey in at least one universe. It connected the city of Olyesti and the other Three Minute Cities to the Time Lord Capitol. (PROSE: The Infinity Doctors)

Behind the scenes
According to Steve Lyons and Chris Howarth's Guide to Gallifrey in Doctor Who Magazine #297, the rail system mentioned in The Infinity Doctors is the Transtube seen in The Ancestor Cell.