looking at what happens with fordable river and "Momentum advance and bonus combats are part of combat, not part of movement".
Imagine if warriors, unit with Leader or cavalry (camel, chariot or elephant included) moving onto a hex with a Baggage wagon after a successful Close Combat. Can Bonus CC, and the mounted units make Special Momentum Advance (+1 hex)and bonus CC?
Triganocerta for example.
Following the clarification on this thread, Cavalry moves into a 'Fordable-river' hex, engages in CC and forces the retreat of defender, can they Momentum Advance?, it's pretty clear (at least to me) that if the unit didn't start on the baggage wagon then they can't take a momentum advance after moving onto the baggage wagon. They can only do a momentum advance off of the baggage wagon if they started the turn on the baggage wagon.
Like the fordable river, "Any unit or leader moving onto a hex with a Baggage Wagon must stop and may move no further on that turn."