With the new tram system in Liège, lots of car lanes were replaced with grassy tram tracks

I'll have to disagree. Yeah lawns are bad and all but your argument is also incorrect.
A meadow has grass and other flowering plants.
A purely grass lawn is still grass.

Oh I understand what you're saying now.

The OP said the grass/lawn is new, so the quickest way to lay down grass is with sod, i.e. rolls of cultivated grass mat.

It's just a start, and the flowers and other diverse plants will come later as seeds naturally disperse onto the new greenways.

It's up to whatever government department maintains the tramways how they want to maintain the greenways.