Comic Sans, tiling background, a pointless start page, you know you're in for some good stuff here. This German website details bogies (or trucks), the parts of train cars that contain the wheels, and how this crucial but easily overlooked piece of technology developed over the decades - and how we came to the modern situation where there are functionally only two main types for freight cars, one used in Europe, one everywhere else.
Freight cars are the main point, passenger cars are only mentioned tangentially. It's also written from a mainly (west-)German point of view; the most detailed pages are generally about things from the German-speaking world and France. But the site attempts to be somewhat comprehensive and brings up many examples from all major railroading areas. I've found it very interesting and informative. If you're way too interested in trains and you speak German or are good with online translation tools, then I can really recommend this one to you all.