The
Utrecht Archives house the former historical collection of the Nederlandsche Spoorwegen, and its' predecessors. Most drawings are high quality black and white scans, and can be downloaded free of charge. More modern drawings however, are not freely available due to copyright.
There's nothing much special in the archives, Dutch engines have never been all that amazing. You'll find designs by Neilson & Reid, Beyer-Peacock, and many more of the prime exporters. However, it's always worth having a look around if you're a nerd like me. Google translate the page for an optimal funnynes to readability ratio.