A spur is a railroad track on which cars are left for loading and unloading. Spurs are also used sometimes for railroad car storage. Spurs are also frequently called sidings or side tracks although a siding usually has a different operational function. Occasionally, the word "spur" is also used to describe a longer secondary route like a branch line which ends without connecting to another through route.
A spur can be single-ended or double-ended. A single-ended spur connects to the through the track at only one end. A double-ended spur connects to the through the track at both ends. A double-ended spur should not be confused with a run-around or passing siding, which is configured the same but serves a different purpose.
A single-ended spur can be a leading-point spur or a trailing-point spur. A leading-point (or facing point) spur branches off in the direction that trains normally travel forward on the through the track. A trailing-point spur branches off in the opposite direction from the normal forward direction of travel on the through the track, requiring cars to be backed in for loading and unloading.
In track design practice, a trailing-point spur is the more desirable of the two because it simplifies how you set out and pick up cars.
There are a few common characteristics of spur tracks which, if duplicated, can make a model more realistic.
Because they see less frequent traffic and trains operate at lower speeds, a spur track doesn't receive the same level of maintenance and quality of materials as the main line. Most spurs sit at a lower level than the mainline. Not only does this save cost on the right of way construction, it prevents a stopped car from rolling out onto the mainline.
Another safety device used on spurs to prevent a rollout is a derail. Derailments come in many forms and are placed in such a way that a rolling car will be gently dropped off the track before it can foul the mainline and cause a larger accident by damaging the switch or colliding with another train. Derailments are an easy-to-model detail and add another step of realism when switching.
Spurs also generally use a lighter size of rail and fewer ties per yard than the mainline. This is easy to replicate on a model by modifying the tie spacing on commercial track sections. When switching to a lighter rail code for the siding, you'll need to use a transition rail joint. There are special joint bars on the prototype as well.
At the other end of a single-ended spur, a car stop will be placed to keep cars from rolling off the end of the track. This could be a steel post or wheel stop or just a pile of dirt or some wooden ties bolted across the top of the rails.
The color and grade of ballast used are also often different. Simply changing the color of the ballast is a quick way to identify different tracks on your layout.
You can add multiple spurs to your model railroad to create interesting operations. Each spur provides an opportunity to pick up or set out cars. This work gives your railroad purpose and can provide much more entertainment than just watching the trains run endlessly in a loop.