When last in the Netherlands we found the doors at Delft station were designed such that a solo bicycle could enter but a tandem could not. I do no know whether that reflects a national standard or is unique to Delft. We did not venture through, but with S&S couplings it would be possible if one took the tandem apart at that point. Whether having gained entry you would be welcome on the train I do not know. One local did suggest to me that the bicycle compartments on the trains themselves and the bicycle parks at stations were rough places to leave a nice bicycle - though I cannot  confirm this. My general experience of the Netherlands is that everything is very cycle friendly and Dutch bicycles tend to be high quality and the facilities for them of high quality too.