https://lists.w3.org/Archives/Public/public-html/2013Jul/0052.html
Re: TextTrackCue discussions from Glenn Adams on 2013-07-26 (public-html@w3.org from July 2013)
https://developer.mozilla.org/en-US/docs/Web/API/TextTrackCue/track
TextTrackCue: track property - Web APIs | MDN
The track read-only property of the TextTrackCue interface returns the TextTrack object that this cue belongs to.
web apistrackpropertymdn