Alan,
Realtracks and Realdrums are recorded audio files. If you have a look in your \bb\Realtracks folder, you'll see what I mean. These files are created by musicians and involve playing lots of chord progressions in a number of rhythmical patterns. The chords and patterns are then mapped by PG Music.
When one enters chords on the chord sheet and then generates the backing, BIAB locates the necessary chords in the Realtrack audio file and then seamlessly splices them together to create a backing.
This is why Realtracks don't play midi.
Regards,
Noel