As you know, RealTracks are recordings of real musicians playing over a prescribed set of chords,
and/or adlibbing some melodic phrases. They don't record note-for-note so there's no way a
RealTrack can follow a MIDI melodist track.

If you have the latest version of BIAB, you might want to check into "Playable RealTracks",
which might be a substitute for what you're trying to accomplish.

Good luck!
LLOYD S