Making up one's mind on just how to theme your model railroad layout is a process that takes in many different thoughts. You have to coordinate thoughts on just what you'd like to portray, (your theme) as well as integral aspects such as era, geographic locations and seasons (and possibly weather).

The first step in all this is to come to a decision on just what you'd like to model. Do you fancy passenger operations? How about mining trains, or city warehouses? How about coastal railways, or grain silos or steel factories?

As you can see there are many, many choices to work through before you go through the time and expense associated with building this great new layout. 

Once you've decided on just what theme you're interested in modeling, you'll need to decide both the time and the place. You probably by this point already have a general idea regarding these aspects of your new layout, but a little more digging is necessary.

The decision regarding the exact time of your layout will have an impact on not only the scenery, (as the town in 1948 probably looks a LOT different that it does today!) but also on choices for rolling stock. One tip you'll want to keep in mind is to check on the availability of trains and scenery models in your chosen era and theme. If not much is readily available, you may end up having to do a lot of the work - either scratchbuilt or kitbashed - and that may or may not have been what you had in mind. You can get as specific here as you'd like. You may choose to model a particular day, event, or time. You are the artist here, so paint what you see!

When choosing specific geographical locations for your themes, make sure you have the ability to see what it looked like then, if you're interested in having it look right. This one isn't really all that hard, as from about the time rail started being part of our lives there was photographic record. Just finding it is the nut you'll have to crack now. Many town libraries will have what you need, and the availability of viewing much of this online is getting better and better. A simple search on the search engines may bring you enough resources to do the trick.

You'll also need to make a choice about the season and weather for your theme. You can really make your themed layout stand apart from the crowd by choosing this area with some careful consideration. Can you  visualize how a New England fishing village rail line might look in early morning fog? Or how about a Western mining operation laboring through a gray day after a heavy snow? (Most modelers don't bother with winter) Or maybe a pastoral Northeastern day as the leaves are turning for a dramatic theme?

The possibilities are endless and your imagination in the season and weather decisions will have a dramatic impact on your themed layout.

Choosing and implementing a new theme is all about making informed decisions and deciding what's more important to you. Are you more concerned with the accurate historical portrayal of your new themed layout or the operational aspects? Either way you go, (or a conscious blending of the two) you'll find that creating a new themed layout is a particularly fascinating pursuit.