Most model railroaders do not realize that their model trains actually incorporate a number of compromises. In the old days when we were happy to have any injection-molded box car kit, we expected compromises --- we knew that the real thing had different ends, and those door claws were a necessary evil for the mass market. However, in recent years, there has been a proliferation of excellent car kits, and most of these compromises have been eliminated. Now, a highly accurate model of almost any prototype is available to almost anyone with a little bit of time and money. Highly accurate except one thing --- the wheels.
Because we spend most of our time looking at models, instead of the real thing, most of us hardly realize that the wheels on HO scale model trains are almost twice as wide as they should be. These inaccurate wheels impact a host of other elements in our models. The frames on steam locomotives must be narrower, and their cylinders wider; the sideframes on diesels and freight cars are almost flush with the sidesills; truss rods and brake rigging get shoved around to all sorts of improbable locations just to accomodate the wheels. The most important impact of the too-fat wheels however, is felt not in the rolling stock, but in the main element of our scenery --- the track.
Those fat wheels with the big flanges require trackwork that is equally coarse. In order to accomodate the thick flanges and narrow back-to-back distances, flangeways on HO standard are more than twice as wide as the equivalent prototype flangeways.
The Proto:scale movement addresses these inaccuracies. The idea is to make wheels and track as close to scale as possible, while maintaining operability.