From the Nightjet website: “From January 2020, the Nightjet will take you on Sundays and Wednesdays from Vienna (8.38 PM / 20.38h) and Innsbruck (8.44 PM / 20.44h) to the capital of the European Union in comfort and environmentally friendly style.”
“After Cologne, the train stops in Aachen, Liège and Brussels. The scheduled arrival time in Brussels-North is 10.45 AM and in Brussels-Midi 10.55 AM. The return journey departs on Mondays and Thursdays from Brussels-Midi at 6.04 PM / 18.04h and from Brussels-Nord at 6.16 PM / 18.16h with arrival in Innsbruck at 9.14 AM and in Vienna at 8.27 AM. Tickets for this connection can be booked from 15 November 2019.”
De Standaard reports a Belgian locomotive will hall the Nightjet from Aachen onwards. NMBS / SNCB is being trained in operating the Austrian rolling stock.