(For those who don't know Brussels, the station is question is signified by the word "Halte" about half way down the right hand side.)
I delved a bit deeper and came up with this slightly more detailed map:
This time the mysterious proto-Schuman is dignified with the title of "Station" rather than "Halte", again a block west of the Rond Point.
A little more digging reveals that the old station of Wetstraat / Rue de la Loi was opened in May 1865, nine years after the track had been laid between Bruxelles-Nord / Brussel-Noord and the station we now know as Bruxelles-Luxembourg / Brussel-Luxemburg, and closed in 1922. I guess that the commuters of the early 20th century preferred to take the tram between the east and centre of the city, rather than the train which loops quite a long way to the north. Me, I rather enjoy sitting in a comfortable carriage for an extra six minutes.
All trace of the old station has now gone. Today's Schuman metro/railway station is a fairly horrible modern jumble of stairwells dotted with fast food stalls, slightly to the south of the area marked on the maps as the Wetstraat / Rue de la Loi station, which is now a patch of ground now overshadowed by the west wing of the Berlaymont. Likewise, the covered market shown in the 1910 maps is now a bleak European Commission building beside the escalators which descend from the Rue de la Loi / Wetstraat to the Chaussée d'Etterbeek / Etterneeksesteenweg.
(NB that the metro station marked near the Schuman railway station here is actually Maalbeek / Maelbeek; the Schuman metro station is marked at the roundabout.)
But we can get an idea of what it might have looked like from the surviving closed station immediately to the north, at the Chaussée de Louvain / Leuvensesteenweg, where two slightly overgrown staircases descend to unused platforms from the main road. The railway at Schuman is at about the same depth relative to the surrounding street level.
It is now a jazz venue.
I am left with one rather minor nagging mystery. The Belgian railfan (ie trainspotter) site I linked to above has the new Schuman station at 5.8 km from Bruxelles-Nord / Brussel-Noord, and the old Wetstraat station slightly further south, at 6.1 km. But an inspection of the maps makes it clear that the difference is certainly in the other direction; the Wetstraat station was north of the road, the bulk of Schuman is south of it. Unless there is some strange convention that you measure the start of the station from the leading edge of the platform? Since today's Schuman is a stop for international services, which I doubt was ever true of the old Wetstraat, I suspect its platforms continue rather further to the north under the Boulevard Charlemagne than was the case for its predecessor. (Or, more simply, perhaps the Belgian railfan site has got it wrong...)