I created my own layout as a specific time and place (late summer 1943 along Stoney Creek, outside of Elizabethton., TN). To me, I never wanted a generic layout.
I wrote a fictional history of the fictional branch line of a real (East Tennessee & Western North Carolina, a 3-footer) and adhered specifically to it. The ET&WNC, in my concept, bought out a struggling logging line in 1936 and made it into a prosperous branch of their system. While designing the layout and buying stuff for it, I kept with the history. If it didn't fit, it didn't get to the layout, pure and simple:
I have a couple of cars that are lettered from the previous line's name (and a few Army marked cars), but the rest represents rolling stock that actually ran on the ET&WNC in that timeframe.
Hey, if you want to do a generic layout to run trains and have fun, that's great. It just wasn't my thing.