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The phrase "givens and druthers" comes from accomplished railroad modeler, John Allen. It refers to the things that can't be changed when making your model, like the size or your room, budget, and so on.  Make a list of these things on a piece of paper or a digital document to help keep these limitations in mind. Use an online document service, like a cloud based one or Google Docs, so you can check your planning elements easily with a phone or tablet. The theme of your railroad includes its purpose, setting, the type of trains or equipment you want to use, and the service the train will provide (like transporting industrial goods or logs). These aspects will influence your planning and how you actually build your layout.  A long coal train, for example, might run through the mountains and carry coal from mines to power companies, steel mills, or residential areas, to provide heat for homes. Locale should also be taken into consideration. Locale includes things like region (East or West Coast, Mountain, etc.), specific towns, or general areas, like the plains of Kansas. The era in which your model train exists will also need to be decided. A model planned for the 1920s would use a steam engine. You won't have modern cars either, in this era. The season you set your model in will change setting drastically. Fall will need to have trees with leaves changing color, winter will need snow, and so on. Grid paper is great for drawing out a to-scale design of your railroad. One of the simplest scales you can use is a 1-inch to 1-foot (2.5-cm to .3-m) ratio, where a 1-inch (2.5-cm) square on your grid paper equals 1 foot (0.30 m) (.3 m) in real life. When planning scale, keep in mind:  The space available for your model. The amount of money you can invest in your model railroad. Larger models will be more expensive. The focus of your model. Scenery focused models will be much different from train focused ones. Your personal limitations. If you don't have good eyes or nimble fingers, smaller models might be unreasonable. Display in railroad modeling refers to how you show off your model. You can arrange ceiling lights to point down on it, direct lighting to it from floor lamps, and so on. Should you be building your model for your own enjoyment, you might not want to devote a lot of time and money to display.  Railroad models built for children and grandchildren might only require a limited display. Adjust your display needs with the purpose of your railroad. If you intend on adding to your railroad and make it part of a larger design, keep this in mind while planning your display. There are many different styles you might use for your railroad track. Some common track designs include a simple oval, a figure eight pattern, and a barbell shape. You should have an overall idea for the plan of your track before drafting your track plan. When first starting out, you'll likely want to keep your train level to minimize more complicated aspects of planning, like grade (steepness) and clearances. If this is your first time building a model railroad, you may want to use a prototype track plan. These are provided by companies and take a lot of the guesswork out of your track planning. Prototype track plans can also be used as an example from which you make your own design and can be purchased online or at hobby shops.  Use grid paper when drawing up your track plan. Keep your track plan to scale with your real model so you don't have to make adjustments later on. You may find that some track plans that you thought would work in reality do not. This is not uncommon, but a prototype track plan will help prevent this.
Assess the givens and druthers. Choose your theme. Determine the scale of your railroad. Identify the kind of display you intend. Plan your track. Draft the track plan for your railroad.