There are more than 10 connections daily. I can't seem to find
the direct Brussels - Maastricht train right now, though, the
best options I see are with two changes.
The SNCB web site is reasonably competent:
On Sunday there don't seem to be any through trains from Brussels to
Maastricht. There is an hourly connection leaving at xx:50 and
arriving two hours later, change at Brussels Nord and Liege, fare is
EUR 18,10 in 2nd class, EUR 24.70 in 1st.
You can buy the ticket online and print it at home (a full page with a
variety of barcodes) but it's the same price as buying it at the
airport train station.
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