Regardless of the scale you are working in, individual model railroad track components fall into six categories: straight, curve, flex, crossing, turnouts, and function tracks. Turnouts are also known as "switches", and in the UK referred to as "points".
Let's look at each of these individually. The actual pieces available in any category will depend on the brand and type of track you select and on whether or not you are using track with an integrated roadbed.
Note: Images may be enlarged by clicking on them.
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With curves, we don't concern ourselves too much with length track. The length of a curve is a function of the scale and two geometric measurements: radius and degrees of the arc. You can see how an interest in model railroading might give kids a head start in geometry class. The image shows the 45-degree curves of various radii. Notice how the smaller radius curve sections fit nicely within the larger radius curves. This enables you to lay parallel tracks around a bend.
Most scales measure curves by radius. O Gauge tracks use the diameter of a circle when identifying the track. The O-36 curve would be the same as 18" radius. Also, while most manufacturers measure curves on the center line, some use the outside rail or the outside of the roadbed.
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Flex track is also exactly what it sounds like; a track that flexes, or flexible track. Flex track is used when constructing permanent layouts and is glued to the roadbed.
Flex track allows you to create curves of any radius and angle desired. When using flex track you need to be careful not to create curves that your trains can't navigate. Flex track usually comes in 30-inch lengths and is cut to the size needed.
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A crossing is where a track going in one direction crosses a track going in another direction. The train can't turn on these sections, it proceeds straight on its current course. Crossings may come in 90-degree, 45-degree, 30-degree, 22-degree and 15-degree angles depending on the brand of track you select. 90-degree crossings are the most common. The photo shows a 90-degree crossing in between left right 15-degree crossing tracks. The straight-through section of the 15-degree crossings is slightly longer than the crossover tracks; this is how you determine which is the left crossing and which is the right.Continue to 5 of 6 below.
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Function tracks are actually a sub-category. They include any track that has a function other than just running or turning the trains.
Feeders and rerailers are the most common kinds of function tracks. A feeder has some means to connect it to your power pack or transformer. This is how your model trains get their power.
Another special piece of track is a rerailer. The rerailer, sometimes made to look like a road crossing, forces any stray wheels back onto the rails as the train passes over it. Sometimes the feeder and rerailer are a single piece of track.
Another common type of function track (not pictured) is a decoupler which is used to decouple cars. These and operating tracks are also commonly used to activate a variety of accessories on O Gauge trains.