Astrid,
You might find that prototype dimensions may be too large for your intended space, even on a diorama.
If you are planning museum quality display with the level of detail you describe, I suggest you select a prototype and follow their specific practice for everything. It will be much easier to do your research in one place and there will be a lot of research. Also, the display will be more accurate prototypically if based on one prototype than if you mix and match standards and rolling stock from a bunch of different ones.
I know standards varied a lot from untreated ties at random spacing and no ballast to class one standards with sharp well maintained ballast. And the appropriate rolling stock and locos and buildings that fit the track appearance shoul be appropriate.
