The Capitol TARDIS was to serve as Gallifrey's new Capitol in the aftermath of the Quark invasion, sometime before the Fifth Doctor was forced to take the office of Lord President. It was constructed in the hope that it would be used to break the non-interference policy which many Gallifreyan's had come to question following the Doctor's appointment.
The TARDIS was described as a triumph of engineering and Block Transfer construction. The Doctor showed distaste for its red colour scheme, suggesting blue would have appeared far nicer.
On the day of the Citadel's opening, the TARDIS awoke after sensing the Doctor's presence. Sharing his enthusiasm to escape Gallifrey it began to reconfigure its interior and exterior, to the confusion and horror of the assembled Time Lords outside. Upon entering its vast console room, the Doctor cautiously but friendlily greeted the TARDIS in an attempt to sooth its confusion following its recent birth, affectionately addressing it as 'Big fella' and acknowledging its excitement at being able to leave. Vorena would later reveal the Capitol's purpose to the Doctor and Leela; To be piloted throughout time and space and continue actively protecting the universe with the full force of the Time Lords behind them.
Disgusted at the idea, the Doctor had Leela destroy the console after programming the TARDIS to leave Gallifrey and trap itself within a time loop leaving it stranded and beyond the Time Lords use. The Doctor lamented this fate for the young TARDIS. (AUDIO: Time in Office)