Time track

A time track was a path through time. "Jumping" a time track could place a time traveller off their intended destination in time. They could even cross the lines of their own destiny.

Physics
Crossing time tracks created recursions that had unpredictable effects. Time tracks behaved differently in different universes. Travel to a parallel universe was not possible as the time tracks didn't meet. (AUDIO: Renaissance of the Daleks)

The First Doctor and Susan Foreman visited the fourth universe which didn't exactly run parallel to N-Space as its time tracks crossed at an angle. (AUDIO: Quinnis)

Instances
The First Doctor jumped a time track in the TARDIS. The TARDIS seemed to only partially materialise, turning the Doctor and his companions into ghost-like visitants in an alternative future timeline in which they had becomes exhibits in the Moroks' Space Museum on Xeros. (TV: The Space Museum)

The Fifth Doctor's TARDIS jumped a time track and was thrown through a time corridor, ending up in an alternate timeline in which the Daleks mutated into insect-like monsters. (AUDIO: The Mutant Phase)

Time track crossing protection protocols were pre-set circuits in TARDISes that were supposed to be engaged all the time. When visiting a place multiple times, this prevented a TARDIS from reaching it by making the TARDIS think it had just been somewhere when it really hadn't. The Doctor's TARDIS's protocol was sometimes engaged and sometimes not. (AUDIO: Renaissance of the Daleks)

When the TARDIS once landed in between times, in a time where this was no time, the Eighth Doctor believed that he and his companions had jumped a time track. (AUDIO: Time Works)

The Ninth Doctor and Rose Tyler chased a Chula ambulance through the Time Vortex. It began jumping time tracks and the Doctor lost it for a while. He managed to find where it landed and believed it to only have landed, at maximum, a month before he and Rose arrived. (TV: The Empty Child)

The Tenth Doctor jumped a time track in the TARDIS that allowed him to visit the Dalek time period before the Last Great Time War. (PROSE: Prisoner of the Daleks)

Mentions
On an Earth-like planet, the Doctor noted that the silence reminded him of Xeros, prompting Vicki to ask if they had jumped the time track again. (TV: Galaxy 4)

The Eleventh Doctor suspected that the TARDIS had slipped a time track when he, Amy and Rory were having several dreams, before discovering it was the work of psychic pollen which had fallen into the TARDIS' time rotor. (TV: Amy's Choice)

Behind the scenes
In Dimensions in Time, a 1993 thirtieth anniversary story not considered part of the DWU by this wiki, the Seventh Doctor was trapped by the Rani, and was jumping time tracks between the years 1973, 1993, and 2013. During this time, he jumped back and forth between his third, fifth, sixth and seventh incarnations, while Ace, his current present companion, became a number of his past companions.