Master Fader tracks
To add a new Master Fader track of the same channel width as the last new track, do one of the following:

Shift-double-click on the empty area of the Mix window below or to the right of any current tracks.

Shift-double-click on the empty area of the Edit window below any current tracks.
If no tracks exist in the project, a stereo Master Fader track is created by default.