We are taking the Thalys train out of Brussels directly to Schiphol Airport, and have subsequently realized that the Thalys trains leave from the Midi Station, not the Centraal one (which is, of course, a block from our lodging). It seems that the Thalys ticket includes a transfer to another station in Brussels, and I've read that others have simply hopped on a train from Midi to Centraal once disembarking from their Thalys train... I would assume we can do the same in reverse, but would love to hear reassurances from anyone who has done this themselves. Thanks for setting my mind at ease!
This is what the Thalys website says: Your Thalys ticket allows you to travel free of charge starting the day before your outbound trip until the day after your return trip on SNCB trains in the Brussels, Antwerp and Liege zones.
So, your ticket also allows to travel to a Thalys station, in your case from Centraal/Central to Zuid/Midi.
Anne, I want to question your first sentence. I think you mean "We are arriving on Thalys in Brussels and immediately going to BRU on the local airport special." I can't believe Thalys sells single tickets from Brussels to BRU, which is what your first sentence seems to say. I guess you didn't buy an open-jaw air ticket.
Your long-distance Thalys ticket is good on any unreserved train (which is 99% of all the trains) between the three Brussels stations. These run many times an hour, probably up to ten. (No research done.) Also, all local train tickets sold for departure or arrival in downtown "Brussels" (not talking about BRU, the airport!) are good to or from all three of those stations. Naturally, I mean the rail company trains. They are not good on the subway system.
If you are going directly to Bruges or somewhere else, you might wish to explore the "Any Belgian Station" Thalys option, but you have to price things out to see if you make out better. Just to Antwerp you would likely not save money. But you'd save a trip through the line at the ticket window!
We were unable to purchase an open jaw ticket, as we are adding a few days onto a corporate trip to Amsterdam--- we were able to show the return trip from Schiphol a few days later was actually cheaper for the company to cover than the original date we were to leave, thus affording us a few days in Belgium on our own dime. Normally, we would fly into one city and depart from another.
Brussels is our last stop in Belgium, and we will be leaving on a Thalys train to go to Schiphol. It seems as if there won't be an issue catching a train from Centraal to Midi in order to board the Thalys.