A derail is a device used to prevent rail cars from rolling onto a rail line and creating a dangerous situation.. It works (as the name suggests) by derailing the equipment as it rolls over or through the derail. The normal position of a derail is in the derailing position (i.e. applied or left on).
Derails come in a wide variety of shapes and sizes. They are often yellow or orange. They are an interesting feature to model because they add operational complexity. Pictured above is a derail (yellow) protecting the Union Pacific main line in San Marcos, Texas in 2013.
Reverse side of derail in San Marcos.
Just as in the real world, using derails in HO scale requires an extra step or two to switching a rail side business. In San Marcos, before the Union Pacific can spot or remove a covered hopper at this auger, the brakeman must use his key and open the derail. By opening it, the actual derail device is moved out of the way so traffic can pass over the derail. (Typically it is on a hinge.) The derail must be closed again once a car is spotted. It is an extra layer of protection in case the brake set via the brake wheel was to not function properly.
This is an obscure feature visually, but of great importance to safe operations of railroads. Two of the great gurus of realistic operations, B. Smith and Lance Mindheim, have taught me that the more you can incorporate realistic operating procedures such as derails, the more enjoyment you can get out of a smaller layout. Though Lance’s modelling is centered on modelling south Florida circa. 2006, Lance’s website is outstanding and a great educational asset — http://www.lancemindheim.com/
Part two of this series will feature HO applications of derails.