A splendid device -- most ingenious in showing "mutations" throughout railway history -- as well as system's growing-and-lessening; re ownerships -- originally, varied rail undertakings, then the State Railways taking over virtually everything, then in recent times diversity once more in evidence, with privatisation (apologies if that's not the exactly-right technical term re the phenomenon).
I note (and, per OP's mention that the creator says as much) that it's the "real railways" system which is featured: the Netherlands' once very numerous multiple-ownerships light railways and -- mostly "steam" -- tramways, on various gauges, are not. (Clearly, the smallish size on-screen of the map would make inclusion of those lines also, totally impracticable.) In that connection -- interesting that in the country's north-east, the rough "triangle" between Groningen, Leeuwarden and Meppel remains throughout, in the interactive map, "blank within". In fact, the "blank" was filled by a number of routes of -- if I have things rightly -- the Nederlansdsche Tramweg Maatschappij, all thereof standard-gauge; a little of same, I believe, having stayed in use for freight until a surprisingly late date (near end of 20th century).
