First is an essay about the Yale Peabody Museum Dioramas
The essay in the above link is not complete, but even in its incomplete state, the information on color theory and perspective is invaluable.
The second link is to a video from the National History Museum of Los Angeles that discusses their diorama and backdrop design and construction.
I like how the artist uses his hand as a mask for airbrushing clouds. Again, note the emphasis on realism. Make sure you watch part 2 for leaf and tree making technique tips (at least for full scale models.)
The consensus I get from both these sources is that a realistic backdrop greatly enhances a diorama. Since model railroads are basically moving dioramas, I believe we can learn a lot from studying this art form. In summary, I am in the photo-realistic (but not photos camp) of backdrop execution. This quote from Wilson essay summarizes my view,
It might be well to remember that the purpose of an exhibit is not to furnish its makers with a vicarious total recall of where they have been, but to communicate some of the lessons and excitement of the location to others who have not seen the site and therefore respond from a totally different premise. In short, let us remember to speak to our visitors and not to ourselves.Finally, I found the following discussion from the Wilson essay apropos. It reminds me of the freelance versus prototype debate we get in model railroading,
Other artists acknowledged that some of this academic knowledge was useful while painting, but that it must remain secondary to the artist's interpretation of the landscape. John Carlson, Wilson's one-time painting instructor wrote:
"Nature is never right" I would modify this by saying that nature is seldom right. The artist must look to nature for his inspiration but must rearrange the elemental truths into an orderly sequence or progression of interests.
This is a theme heard repeatedly by landscape painters past and present. According to prevailing theory, merely copying nature produces inferior results. They would take their studies made on-site back to their studios to rework them into larger canvases. Wilson, if he ever made a larger painting from one of his studies, he would enlarge it using a grid and make a direct copy. The on-site paintings were documents of landscape phenomena observed, color notated, contours found, atmosphere harnessed, light translated. This was why he painted; there was no further need for refinement. Wilson, in his quiet manner, challenged the claim that the artist can inspire better results by sensitively reworking the scene in front of them. Wilson, in contrast, chose the landscape over the artist.