Why do you want a layout?
So, you've decided to build a model railroad but you don't know how to begin? Start by asking yourself an important question: What drew you to model railroading in the first place? Understanding your reasons for wanting to build a model railroad can help you develop a satisfying design. Some sample reasons and their design implications are listed below.
"As a child one of the best things about visiting my grandparents was sitting on the hill with a glass of lemonade watching the trains go by." If that memory is one you'd like to revisit, you may want to design a railfan's model railroad. You might create one visible scene with a lots of hidden tracks. The hidden tracks provide a place to park the trains that will run through the visible scene. Model railroaders call the unseen portion "staging" since that is where we hold the trains that are waiting to come "on stage."
"My mother just gave me all my old trains; now I need a place to run them." When we were kids, we ran our trains around an oval of track. Would you find that oval interesting now? Perhaps you should consider building a more complete transportation system on which to run the old equipment. Or would you, perhaps, be better off selling that equipment to a collector and starting over with today's improved equipment? Or you might build display shelves for those valuable memories. If you want an oval of track on which to display those trains, reread the opening scenario as the railfan railroad lends itself well to this desire.
"I love riding the train. Always have, always will." You may want to model passenger service rather than freight trains. Decide whether this means lots of commuter trains or a long mainline with few trains but lots of stations or, perhaps, a coach yard where head-end cars are switched out and diners and sleepers are serviced.
Replicating the Work
"I used to hang out at the tower and watch the dispatcher work; I'd like to try doing what he did." Recreating this experience requires a large model railroad with lots of staging. It must have a long main line with enough sidings for meets and passes to provide some challenge for the dispatcher. You will need either a huge room or a multiple-deck layout. You also must have enough friends interested in running trains to provide the traffic to be dispatched.
"I found an industry that fascinates me." Depending on the industry, this may be enough to be a whole model railroad in itself. For example a large flour mill may have a receiving yard and a departure yard where the main line trains drop off and pick up cuts of cars. The industry switcher may move loaded cars from the arrival yard to the grain testing area, then to the unloading area, then to the departure yard. It also may move empty cars from the arrival yard to the clean out spots, then to the loading area, perhaps to a weighing area, then to the departure yard. Other large industries have their own "interior" car movements.
It's my World
"I have built and detailed many locomotives; now I need a place to show them off." The best model railroad to meet this need may be an engine service facility with its many visible tracks. The engine house might be small or made at least partially of clear materials since you don't want to hide the locomotives from view. There could be auxiliary tracks such as fuel tracks, wash tracks, ready tracks, etc. The locomotives could then be hostled from the yard arrival track to the service facilities, to the ready tracks, to the departure track.
"I like getting together with people who can talk trains." You may want to join a club where each person can contribute according to skills already developed or learn new skills. At the very least build a large enough model railroad to require several operators. Leave space for a crew lounge for great discussions when the operating sessions end.
"All day at work I take orders; when I get to the railroad room I want to be in charge." Any non-club model railroad will let you make all the decisions as to what will be included and how it will be done.
Stop, Look, and Listen!
If several of the above scenarios seem to fit for you, try developing a plan to include your favorites. If none of the above scenarios fit, stop, look, and listen! Pay attention to what you pay attention to. Which articles do you turn to first in train magazines? Where do you spend the most time on a layout tour? Which scanner conversations catch your ear? Which jobs do you sign up for at operating sessions? Which contest entries do you linger over longest? What do you photograph when you are rail fanning? Soon you should be able to identify your interests. Use that information to design a railroad you will find satisfying. Think of a coin toss; what's important is not how the coin comes up but how you feel about the results. When your railroad feels right, the design is right.