the 4000/4200 we call it the koploper (head walker)
the 2400/2600 we call it SLT (sprinter light train)
and the 8600 etc we call it v-irm verlengd interregio materiaal (extended interregional material)
Indeed I am aware of that, but outside of the Netherlands if you tell someone you've been on the Koploper, the SLT, VIRM or the Hondekop (which on looks alone is my personal favourite ) they'll be I figured giving the class numbers would be easier for people to look up + rail geek = listing by stock class rather than common name (although I refer to them by those in general conversation)