There was some American astronaut who, in some slightly cliched way, pointed out that borders are not visible from space. I think Belgium was a country which was effectively invented by the British in cahoots with the French and its a truism that borders themselves are a purely human construct. Therefore, when Schengen was agreed, a border-based infrastructure, complete with jobs and buildings and "entry procedures" became instantly redundant. Of course, by the same token, there wasn't any money to physically remove the actual buildings, so they remain to this day, reminding us that Belgium, whilst an entirely man made entity, still, kind of, exists as a notional "nation". Mind you, one wonders whether anyone ever actually tried to enter Belgium illegally, that is, did anything controversial EVER happen at this border, or did it just serve to assist in the accumulation of pension funds for Belgian officials (latterly replaced by that administrative Behemoth, the EU)?