I reserved seats on trains Brussels Midi to Stockholm Central.
(1) How do I buy the actual ticket
(2) What happens if I don’t use the reservation?
On what website did you that that? For what date and time?
Normally you buy a ticket first, and buy your reservations together with the ticket. Sometimes you buy a ticket and do not buy a reservation at all. And sometimes you buy a ticket and buy the reservation later.
But buying reservations first is an odd way of doing things.
Now as to your questions:
- The ticket you should be able to buy at the same place you got the reservations.
- If you do not use a reservation it lapses. If a reserved seat on a train is not occupied within 15 minutes of the departure station then the seat is freed (in German trains it will disappear from the reservation display) and anyone can take that seat.
This is a very long trip, so I wonder what trains you believe you booked.
I too am curious about the route. From what I can tell, it would be Brussels to Cologne, then Cologne to Hamburg, then Hamburg to Stockholm. A DB reservation would cost 5.20 EUR, only covering the the Brussels to Hamburg segment. From Hamburg to Stockholm is by one of 2 night trains which have reservations included with the tickets. One is operated by Swedish national carrier SJ, and the other by Swedish private operator Snaelltaget.