I was looking into the differences between the tracks contained in the '*.geant.root' files (which I call "geant tracks") and the tracks contained in the 'StMuMcTrack' branch of the '*.MuDst.root' files (which I call "MC tracks").  Below is the particle ID for 3 cases: geant tracks, all MC tracks, and MC tracks with 'IdVx == 1' (i.e. their vertex of origin was the primary interaction vertex).  The colored text in the label indicates the total number of tracks for each case.  The histograms were normalized by the number of entries.

This indicates that the '*.geant.root' files contain all the tracks produced at the generated primary vertex, and nothing else.  By design, everything which doesn't satisfy 'IdVx == 1' has 'IdVx >= 2' which indicates that it was produced at a secondary vertex.  This is important to keep in mind as we keep digging into the embedding sample...