Choosing a theme for your model train layout will help immensely with the planning. The theme is based on the era for your model train set, your setting, and your operating style--and choosing a theme that appeals to you is key to enjoying the hobby.
In the model train hobby,
a lot of what you'll do will flow from the theme that you've chosen for your model railroad. From scenery to rolling stock, from track to power supplies, everything will depend on your theme.
When it comes right down to it, theme is all about what you want to do.
Choosing a theme is all about:
an era
a setting
an operating style
Model Train Eras:
There are three main eras for model trains: steam, modern, and transition.
1. The Steam Era covers everything from the Wild West (1870s) up to World War II (1940s), but for most people the steam era pretty much covers the 1930s to 1940s. The visual appeal and nostalgic elements of intricate models of steam engines pulling a train of rail cars are tough to beat.
2. The Modern Era generally means today's trains, although it could conceivably cover anything from the 1960s to today. The modern era can cover everything from ultramodern diesels pulling intermodal trains to commuter trains and everything in between, but for most people a modern era train layout will show locomotives, rolling stock, buildings, and scenery that you would expect to see on the rails today.
3. The Transition Era is that period between the 1940s and 1950s when both steam and diesel locomotives travelled the rails as steam was being phased out and diesel was being phased in. The transition era is best for modelers who want to run both steam and diesel engines on the same tracks true to prototype--in short, modelers who want the best of both worlds.
While you're considering your choice of era, you may want to factor in the availability of material for the different eras at the hobby and online retailers. There tends to be much more modern-day rolling stock and model kits available to the consumer than steam-era, and often at much lower prices as well. Even train sets, which can be a great source of lower-cost locomotives and rolling stock are usually modern-era. Even the second-hand market, such as eBay and flea markets, tends to have a lot more modern-era items available.
You will also want to consider how much of a model-railroad purist you want to be. Although many model railroaders start out being happy to mix elements of many different eras, such as blending steam-era locomotives with the diesels of today, many people in the hobby tend to become more purist as they grow in skills and enthusiasm. Call it "prototype fever"--modelers who catch it end up becoming more and more interested in modeling a particular era to the exclusion of others. They find that their 1930s-era logging railroad just doesn't look right with a 21st-century diesel locomotive running on its tracks. So, if you end up with a layout that isn't set in the era that you want, then you'll have to pull out and replace the items that don't fit, which will cost you both time and money.
Model Train Setting:
After you've chosen an era, you can start looking at the setting for your layout. With the Steam era, you can choose from a wide range of setting including, for example, the Rockies with lots of mountains and trees. Or you could choose the Prairies with lots of wide-open spaces broken up only by tiny farm towns. Or, there's the Midwest with its bigger cities and heavy industry. Or you can choose the Coast with small fishing towns and big ports. The settings are pretty limitless--and universal. Although the regions I've listed are more suited to North American railroads, the same types of regions exist pretty much anywhere in the world that rails have been laid.
Now, while you may want to try something pretty unique for a layout, keep cost and availability in mind. Narrow gauge layouts, for example, while stunning to look at and operate, demand a lot of time, effort, and money to set up. More mainstream settings are cheaper and easier to model for beginners. When you're starting out and learning the hobby, avoid spending too much money if you don't have to.
Operating Style:
The most important factor in choosing your layout theme is your operating style.
By operating style, I mean the aspects of the hobby that you really enjoy the most. If you really enjoy scenery construction, then your model railroad should give you lots of opportunities to show off your scenery. On the other hand, if you really enjoy coupling together trains and switching, then you should really look at a yard module, with lots of track, rolling stock, and switches. And if you enjoy operating your model railroad the same way that a real railroad would operate, and playing with items like schedules and "fast clocks," then you should look at putting in lots of model industries, sidings, and destinations for your trains.
Now, I have talked about cost in choosing an era and a setting for your layout, but I'm going to suggest that you give cost less priority when it comes to operating style. Your operating style is the reason why you're in the hobby. Plaster's cheaper than track, but scenery heavy modeling may not have the same appeal to someone who is really interested in setting up and operating a railroad yard. And if you're not going to enjoy a particular operating style--then why do it?
Hobbies are supposed to be fun. Yes, watch your budget when you're starting out but at the same time make sure that the reason that you got into model trains in the first place is also your guide for choosing the theme of your model train layout.
Now, once you've considered the era, setting, and operating style you want, you have pretty much narrowed down the theme and you can move on to more detailed planning. Keep working on your theme and refining it until you have a good vision of what you want your model railroad to be.