Lionel Sound Activation Button 6-5906.
Sound Activation Button is used to activate an electronic bell when the transformer only has a whistle/horn button. You will need two if your transformer does not have a whistle/horn button. One button will activate the whistle/horn and the other will activate the bell. Comes with an installation manual with wiring diagram. Does not work with a Lionel 6-4690 Type MW transformer.