The Doctor's first TARDIS

A Type 50 TARDIS was the TARDIS which the First Doctor used prior to his life as a renegade Time Lord. The First Doctor decided to steal another TARDIS, a Type 40 when he finally departed Gallifrey with his granddaughter Susan Foreman. The Type 50 TARDIS escaped Gallifrey on its own. (AUDIO: Prisoners of Fate)

Not to be confused with the TARDIS the Doctor initially chose to steal before being persuaded to take another by a splinter of Clara Oswald (TV: The Name of the Doctor).