Here are some excellent suggestions sent in by Harold who is a member of my popular online Model Train Club:
Most of us have far more cars (rolling stock) and locomotives than we can possibly operate at once. One option is to simply store excess cars and locomotives in boxes or on display shelves or cabinets. It is possible, however, to store all of your extra equipment on tracks in a yard that may, or may not, be part of the visible operating portion of your layout.
Here are some tips if you are planning on building a rail yard:
1. Try to incorporate at least one staging track with it connected to the main track at both ends. If you have room for two, then even better. The purpose of yard (apart from storage) is to collect incoming cars, rearrange them and get them on trains that will take them to their destinations. The key is to have enough space to do this efficiently.
2. Try not to cram too many tracks into a small space. If the tracks are too close together it could cause a domino effect if a car derails.
3. Always plan an escape route and avoid having spur tracks that could trap your locomotive behind a line of freight wagons.
4. Keep your yard design simple. A complex design can lead to more potential problems even with a simple mistake.
A yard should be relatively easy to get in and out of. Keep the design simple and the operation will be simple too.