Can you transport foldable bikes in the train and bus in Belgium? If yes, does that have a cost or is it considered luggage?
I checked the Belgian railway site for you and they mention that folding bikes are free of charge.
The Flanders bus company 'de Lijn' has a mention of bikes on their Dutch language site, in short you can take a folded bike for free if you do not bother other passengers but no full size bikes at all.
Als u de andere reizigers niet hindert, mag u wel gratis een opgevouwen vouwfiets meenemen.
I have found a site for buses in the Walloon area of Belgium, but as I do not speak French nor German, I am stuck when searching for folding bicycles.
Loufylouf read in the site and tells us that the rules are the same as in 'de Lijn' area. You can, as long as you don't hurt or bother someone else, transport a folding bike in their buses.
That leaves only the Brussels area, for which I found this site.
When completely folded, folding bicycles are not included in the present rules because they are considered to be packages.
So they are allowed. Even though not mentioned in these rules, it is to be assumed that bringing folded bikes on busy buses is not appreciated and might be forbidden, as all huge packages might be unwelcome.