TrackMerging is a simplified version of TrackGrouping. The basic idea is to allow a single "song" object to be shared by multiple "album" objects. A "song" object contains a collection of "track" objects, including their respective titles, durations, track numbers, and PUIDs. This would clear up a significant amount of redundant data and allow us to pool similar PUIDs into a single object such that there would be absolutely no PUID collisions between the objects. Implementing this concept will be one of the many incremental steps toward the NextGenerationSchema goal.
TrackMerging is currently being worked on by LukasLalinsky in the following branch: http://bugs.musicbrainz.org/browser/mb_server/branches/track-merging