Transtube

The Transtube was an underground train on Gallifrey. Its central station was under the Time Lord Capitol.

Before the outbreak of the War in Heaven, when the Edifice began to turn Romana III's Gallifrey into a crumbling and superstitious shadow of its former self, the system had fallen somewhat into disuse. The tunnels were accessible from MidTown. (PROSE: The Ancestor Cell)

In one state of reality, the city of Olyesti and the other Three Minute Cities were connected to the Capitol by rail, (PROSE: The Infinity Doctors) meaning the Transtube. (PROSE: Gallifrey: A Rough Guide)

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.