Note that the horn/whistle is shown as a negative (ground) input connected to terminal H1. A positive horn/whistle input should be connected to terminal H2.

Again, the horn/whistle can be operated by means of a switch or pushbutton mounted on the loco, by means of a reed switch operated by trackside magnets or by a simple wireless device such as a door chime or garage door actuator. In battery powered locos, the horn/whistle works on a momentary basis ie. it starts when you close the switch and stops when the switch opens again.