Home Made Derailer - Model Railroader Magazine

I thought that the one in the MR article actually functioned as a wheel stop rather than actually acting as a derailer, even though it was called a derailer.

Anyway, do you actually want something to derail the cars, or are you looking for something inconspicuous to keep the cars from rolling?  There have been a couple references to folks who have used a nylon brush bristle to act as an anti-rolling device.  The bristle is stuck between the rails and will stick up enough to engage a car axle and prevent rolling.  It is also flexible enough to allow the cars/engines to pass over when you want them to.  The other option I've heard of is a mechanical device, usually home-made, that will shove a metal pin up from between the rails when you want the car not to roll, and then retract the pin when you are ready to go.  Obviously, the metal pin will give you the derailment you mention if you forget to retract it.