I am considering booking a ticket to Brussels, but using the air/rail option to Brussels Midi (ZYR), flying from Aberdeen via either AMS or CDG.

Some websites show this as a bookable option, ie Opodo, but others Expedia (for example) do not - any idea why? Could it be because they use a different GDS provider?

I'm asking more out of curiosity than anything else, though the flight+hotel deals via Expedia are sometimes the best option, and it's slightly irritating I can't book via their site.