And in passing, don't think that roller bearings are pure rollers, either, because many types of rolling-contact bearings have plenty of sliding friction, tapered rollers being one of the worst offenders. IMHO, what seals the deal for roller axle bearings on rail vehicles is that they have a secure environment maintained by effective and reliable seals; and because they are cartridge assemblies, the environment stays secure (usually) until they're swapped out. (In addition, the much smaller static friction, compared to plain bearings, saves lots of fuel.) In stark contrast is the plain axle box with a non-secure hinged lid and a simple, ill-fitting rear seal that often wears out and is difficult to replace. As a result, this very dirty environment is the cause of most failures, in my opinion.