This is one of my favorite LEGO sets. It is rare and hard to find in working condition and it is very old like me! From 1968, it uses a mic (actually brick with built in microphone) and a electronic circuit brick (clear so you can see the circuit). It connects to the motor and battery pack (common for that time) and then you can start and stop the train by blowing on the whistle! We tried the Thomas whistle but it is not high pitched enough (will upset dogs with the LEGO whistle)!
Trained as a Physicist - I have worked in telecommunications for the last 18 years as a problem solver and 911 tech. I love trains and LEGO and my YouTube Channels are Train Tsar Fun, BrickTsar, and BrickTsar Toys. Trains is a feature on all of those channels.