Another key component of the design is which time period the model is intended to represent. Generally, these are either steam-era, transition-era (where steam and diesel locomotives are both running), and Diesel-era. The transition period is usually considered to be the 1950s and 1960s.
For me, I think steam locomotives are more interesting than Diesel locomotives but I like the more modern rolling stock and scenery options. I have decided to aim for transition-era modeling nominally 1960 but do not intend to model a specific prototype railroad. The setting will be the Western US, around Denver, and use mostly Union Pacific (UP) locomotives and rolling stock. I will make sure to use period-appropriate items, i.e., locomotives that were part of the UP roster around 1960.
One consequence of the defined time period is that this was prior to the development and use of containers for shipping the vast majority of freight. I feel this is actually a benefit because boxcars are more interesting and varied that long trains of containers on freight cards.