Do you really need architecture principles? If they state the obvious or doesn't help you design a solution, why bother?

We go back to our model railroad example where we need to do some high-level decisions. Which principles do we need to guide our city planning in small scale?

I listed seven questions to be resolved before you start, but are they principles, or related to principles?

Size
Scale
2-rail or 3-rail system
Analog or digital train control
Timeperiod, e.g. era and country/region
Prio of building landscape or running trains, or both.
Budget, or more time.

Is size a principle? It's very difficult to create a layout larger than your available physical space, unless you make modules to switch between. Thus no principle.

What scale to use? If you try to match different scales, they don't fit together, so you need to select one. Devils advocate here. You can have two non-connected tracks on a layout with different scales. In the real world you have narrow tracks, so some variations are allowed. Buildings are not always in scale 1:87 but a tad smaller. If you mix H0 with N-scale, the result will be less real. Neither a principle.

2-rail or 3-rail system? You can't run locomotives for 2-rail on 3-rail tracks, or vice versa. But you can run cars manufactured for 2-rail on 3-rail systems. More option with 2-rail system and more realistic, but less simple. No principle either.

Analog or digital train control is also a choice to make. You can't mix track sections for analog and digital, so either of one if you don't do two different layouts as different gauge. Not a principle.

Time period and country/region is a bit different. You can mix time-periods if you want, and there are creative ideas how to make this realistic. You still can see old steam engines running on the tracks as today. Trains and wagons also cross borders. Time period is not a principle per se, but strive for realism could be a principle

Prioritizing modelling or running train, including level of ambition would be different, depending on the person who is paying and her objectives.

Budget and time are not principles, they are constraints in the same way as size of your layout. You need to manage them.

What could then be the principles that help you with the design? Here are some suggestions based on my trials the last year.

First of all, have fun, this is a hobby.
Strive for realism, not mixing things that wouldn't be plausible in real life.
Avoid symmetri is another principle related to realism, as nature is not symmetrical.
Have a reason, a purpose, for your locomotives and wagons.
Design for accidents with rolling stock and other problems.
Build before buy is an option today with the arrival of 3D-printing.
Build in modules is another principle that makes your model railroad more scalable and flexible.
Build you landscape step by step, not a big bang.

All these suggestion of principles help you with the design of the layout and what to buy for your personal taste. You may not agree, that's fine. But it's good with principles that are relevant for you and your own city plan.

Disruptive architecture