text: Data in OSM is correct.
  The reason is that the designer of the "Transport Map" offered on the main OSM website made a choice to ignore route numbers for trams, metros and trains.
  Check out this great website here, they take the same data but they present every mode with its own colour, and they show the route numbers: http://openptmap.org/?lat=50.8508&layers=B0000&lon=4.3557&zoom=16