Line 9 services begin at an interchange with metro lines 2 and 6 at Simonis, sharing tracks with tram Line 62 as far as Miroir before diverging onto a new 3.3km seven-station branch to Brussels University Hospital and Arbre Ballon, which has a three-storey underground car park with spaces for 199 cars.

Construction began on the €76m project in 2015.

Services operate at six-minute headways at peak times and every 10 minutes off-peak, with a 15-minute-interval service in the evening and at weekends. The journey time between Simonis and Arbre Ballon is around 15 minutes and Brussels Transport Authority (Stib) is using modern Bombardier Flexity LRVs on the new line.