Theme and Variation
Or - How Choosing a Theme and Era Early Can Save You Money
All real railroads share a similar purpose - To get people and/or goods from point A to point B, and (hopefully) make money doing it. To the novice, there seems to be a bewildering number of choices on HOW they did it, and WHAT they did it with. If you're like most folks starting out, you probably haven't thought much about this yet, and have just bought whatever caught your eye that you could afford..... There's nothing "wrong" with that approach, as long as you stick with it. But if you later gravitate towards a theme that doesn't include the stuff you already bought, then you have some pretty pricy dust catchers, also known as 'shelf queens' to look at or sell. (usually at a loss)
That's why a little more thought early on can often save you a great deal of your finite hobby dollars - in a very real sense getting you 'more' for 'less'. So without further ado, let's look into the options:

Choice #1: Standard or Narrow Gauge? - Standard gauge means 4' 8-1/2" between the rails on the prototype (full sized) version. There are also many narrow gauge choices, and even some rarely modeled broad gauge ones as well. Even though in large scale these will all use the same track gauge, the varying scales don't often look very good together, even though (in theory at least) they will all physically run together.

Choice #2: Era? - For simplicity sake we'll divide over a century of railroad history into 5 rather broad, and somewhat overlapping catagories; A. Early steam - 1825(or 1880 with most commercial offerings) to1900.   B. Late steam - 1900 to 1950   C. Transition (both steam and diesel) 1945 to 1960   D. First and 2nd generation Diesel - 1945 to1970    E. Modern - 1970 to present. Just for the sake of completeness, I will also mention that there are electric railways with various eras as well. I'm simply not well versed on them.

Choice #3: Purpose? - Mainline, Branchline, or Industrial? A Standard Gauge Class 1 Mainline offers impressive long trains, high speed operations AND eats a lot of real estate very fast. An interesting and fun Narrow Gauge Quarry layout can fit on a shelf. In between there are lots of choices, niches, and most of all; sizes. If you don't have much room, then a simple Industrial theme (Quarry, Logging, Mining, etc) might be your best option. If you have middling space a larger Industrial or smaller Branchline can provide a lot of interest. If you have acres to work with, then go right ahead and build whatever you like. You don't need me, lol!

Choice #4: Primary Commodity? - Passenger, Commuter, Freight, Mixed Traffic, or Commodity Specific? What good are those cool looking cars that you rarely run? Log trains tend to be exactly that, logs.- Perhaps with a beat up combine sometimes tacked on the end for the woods crew. Coal, iron ore, slate, oil (when close to the fields), livestock, etc were very often shipped in unit trains as well. Freight trains carry 'stuff'. Passenger trains carry people, mail, and express goods in specially equipped cars. Mixed trains with both goods and people were mostly only found on sleepy branchlines. Commuter trains were/are short passenger trains that moved folks from the outskirts to the city centers. Having a couple different train "sets" of different types to run as the mood strikes is what most folks do. Not always an option if you have limited funds... or storage space.

Choice #5: Region? - This goes well beyond just the roadname. Not only does the scenery change, but often the equipment and architecture as well. Some things that are quite common in one place may be scarcely or never seen just a few hundred miles away. In the US the broad catagories most often used are: Northeast, South, Midwest, Rocky Mountain, Pacific Northwest, and Southwest. Many eventually choose to narrow it even further to a specific state or locality. Model railroading, book learning and research will often go hand in hand the deeper you get into it. IF you choose to.

There you have it. The choices are all yours and there are no wrong answers (well, maybe logging where there are no trees?). AND there's no reason to stick with the theme you choose now if you outgrow it or get bored. Sell or trade off the old stuff to buy what you want now. You'll come out behind financially, but ahead in terms of satisfaction.