Modeling Long Ago
From 1900-1950 is a Halcyon period of steam engines, a variety of
operations, shortlines, narrow gauge, passenger operations.
Do you like brown boxcars? Do you hate Helvetica and anything jet set?
Do reliable diesels seem too easy to you? Do you like squinting at silver
nitrate negatives to try and discern what color to paint something?
Modeling Your Youth
Those good old days when you had plenty of time to watch trains and no
money to buy them.
Does it give you a concept you can work what you want into?
Are you in the demographic that manufacturers are catering too?
Modeling the Present Day
It is what you are appreciating now. You can go take a nice color
photograph of it, or better yet, someone else already has and posted it on
Limited switching and shortlines. Most trains 3+ engines and 100+ cars,
difficult to model on a modest layout. Really easy to just spend your way
into a layout.
Modeling Your Neighborhood
Easy to get materials
In your basement is the same as outside.
Modeling Far Away
Tropical port / city? Icy Alaskan tundra? South American mountainside?
May look nice, but have limited operation potential, and be harder to get
accurate detailing of.
Exactly what you want
May be hard to keep coherence when you keep piling things on.