I don't see there being anything wrong with a diesel locomotive, or an electric for that matter. they run on rails, played, and still are playing, major parts in the history of America's railroads, and came in an amazing variety of shapes, sizes, colors, and styles. In my personal opinion, many of the older units are just as fascinating as many steam locomotives. Plus, the idea of the standard smoke belching filthy coal burning locomotive seems a little blase' to me. If you only focus on that, you block out massive amounts of historical every day railroading. Even the Norfolk & Western doesn't really fit your description, I mean they had streamliners with a red racing stripe! The Union Pacific had Greyhounds, oil burners, and even a streamlined Armor Yellow and brown Mountain. If all locomotives fit the same guideline, railroading would be incredibly boring. Utilitarian wasn't always the way it went, and straying from the bland produced some amazing machines. I personally like seeing a tourist locomotive not necessarily decked out, but at least well kept up with some obvious care being made towards its appearance and up keep. It may not be exactly how it was, but it's how it is now...plus, it's only paint. So that's my long-winded opinion, and you of course are entitled to yours. It's an interesting topic for discussion.
Thanks, Taylor