There is a pretty well known reliability curve for a typical electronic product. Note that this is only for a properly designed product, poor design decisions will greatly affect this statistical curve. Properly designed electronics with the entire operating envelope within all the component ratings should exhibit a similar failure characteristic. Obviously, the specific component types are key in the calculation, as well as the MTBF of the individual components. If you pick a capacitor that has an MTBF of 1,000 hours, and all the other components are rated at100,000+ hours, you can still count on an MTBF figure that's pretty low.
You will note that there is a higher rate of failure early in a product life, infant mortality. This is why high reliability products have a factory burn-in cycle. For much of the avionics I developed, there was a required 168 hour burn-in and then a complete functional test before shipment. That would move the failure rate into the Useful Life area of the curve. Since the MTBF of a product is based on the field experience, anyone that needs to maintain a specific MTBF figure is well advised to have a production burn-in cycle. Obviously, the model train folks don't feel any such compunction. 
