Volume is a rotatable knob, usually a slotted one under the frame. That switch selects fwd or rear coupler.
Once the train is running, if you go to neutral, raise voltage full and press the whistle/horn button you will hear the coupler arm. Release button, lower voltage to a more reasonable level and go back to running the train. The next time you hit the whistle button, the coupler you selected will fire and open.
This was older tech, so only one at a time could be operated for MTH. You can rearm and refire over and over. Go to MTH site and pick a diesel from before 2000 and you will get a PS-1 instruction. All effectively work the same. If you had smoke there would be an on off for the smoke. PS-1 smoke runs off track power. G
