The Track Matte feature has been greatly improved in the latest release. The layer can be anywhere in the timeline and can be used as the matte for multiple layers. Make sure you have both the Modes and Switches columns visible by selecting the first two icons in the bottom left corner of the Timeline panel, then experiment with the new UI. To learn more about the changes type Track Matte in the Search Help field in the top right corner of After Effects.
There are also several threads on this forum on the subject.