Let me try to answer the question directly than just "Its YOUR RAILROAD" (I see someone modeling Mars Aliens invading a model railroad destruction scene...yea its "MY RAILROAD".... sticking to a period makes the scene look specific and gets the look for that period, but if you get inspired by differrent eras having a layout thats a little more generic helps, but you realize there are a lot of old towns around still with original old buildings, you would think you were still in the mid fifties or whatever and then here a modern Diesel with doublestacks flies thru town and your in culture shock. Its a matter of self disclipline, understanding your period you want to model, I owned some White Pass and Yukon DL535 narrow gauge diesels brass you know for awhile, then I sold them because they were out of my period...then I realize how rare these critters were..DOH!!! (kick rear) I have a lot of steam and 50's era diesels, but then I own some DD40's and some modern equipment, so maybe just for jollies I'll sneak them on the layout hardee har har....they'll never notice..hee hee hee
