This is small, powerful amplifier that can be used to increase the output of the ERR Railsound kits in your engine, or other sound sources. The physical size support applications in most scale sized applications. 1.5″ long by 0.6″ width. Supply is physically larger when used with O Scale applications due to capacitor size. Not quite as loud as the prototype, but you can set the volume as desired! Great feedback from customers.