Apple has yet to update its official list of locations for which Maps offers Transit data, but several tipsters contacted MacRumors to confirm the information is already live.
The coverage includes train, tram, and bus routes in the main cities as well as other funicular connections around the country. Belgian connections to nearby connections abroad also feature in Apple's coverage.
Google Maps has supported transit directions for several years now, but Apple Maps has been adding transit data since 2015 and is steadily catching up with detailed routing information.
(Thanks Bernd and Noah!)