We did Brussels and surrounding cities with a 9-month old and took the UppaBaby G-Luxe umbrella stroller.

We took local trains to nearby cities (Antwerp, Ghent, Brugges, Oostend) and ended up using an Ergo carrier when walking within or between train cars and when boarding the train. The umbrella stroller couldn't fit down the aisles comfortably and it was faster to use the carrier than hoist the stroller with baby inside it.

For the metro, we used the lift when available (not all stations had one). Beware of pickpockets who pose as 'helpful folks' especially with a baby.