Brussels airport train to Brussels central station

Purchase tickets at the ticket counters, and the entrance to the station. They accept cash and credit cards. Although if you use magnetic strip card the clerk has to fill out a form, which can be time consuming. Best to use cash.

Thanks for the tip. Are there any self service ticket kiosk?

You're welcome. There are ticket machines but you have to make payment with some sort of private debit card only available in Belgium. It doesn't seem like even the locals use them.