Pretty much. The only infrastructure you get between say Netherlands and Belgium is a signpost, but between Schengen and the customs union there's no reason not

Pretty much. The only infrastructure you get between say Netherlands and Belgium is a signpost, but between Schengen and the customs union there's no reason not to. Between UK and Ireland it's less than that. On the Spain Portugal border there is some infrastructure, at least at some points. One of these is a zipline over a river. Between Netherlands and Germany the border actually runs through some public buildings. Are there any buildings that straddle state lines?