Transtube

The rail was a transport system on Gallifrey in at least one universe that 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 DWM, the rail system mentioned in The Infinity Doctors is the Transtube seen in The Ancestor Cell.