The restrictions will be because of the loading gauge, the safe envelope around the train in which it can operate without hitting anything. When a train moves there is latitudinal and longitudinal movement [up and down and side to side]. Also when a train goes round a corner, the ends of the vehicle can swing out.
The M6 coaches are long and tall.
Some lines will have a restricted loading gauge due to who built them. The lines in the south of Belgium may have been built by French Companies and the loading gauge in France is restricted. The lines built in the north of Belgium may have followed Dutch and German practice where the loading gauge is bigger.
https://nl.wikipedia.org/wiki/Omgrenzingsprofiel
The loading gauge problems are especially difficult to solve where there are lots of tunnels. Most of your tunnels in Belgium are in the south and these these will have been dug out to the French standard. Having a restricted loading gauge is also cheaper to build as the smaller the tunnel, the less you have to dig out.
Most of your rolling stock in Belgium can work on any line in the country. With something like a M6, being so tall and long, the envelope in which it operates will be larger than for a M4 coach, which is smaller.
Sorry for posting in English.
Iain