If the word “train” didn’t grab your attention, you’re probably not the target audience for this project. If it did, then there is no need to explain why [AeroKoi] has come up with a modular 3″ train whistle, available on Thingiverse and well explained in the video embedded below. It’s a train whistle. Who doesn’t want their own train whistle?The great part of this design– aside from requiring less air than his earlier, larger-diameter iterations–is the modularity. With adjustable bell caps, you can alter the size of the resonant chamber and thus the notes the whistle plays on the fly. That’s the key difference between a train whistle, as used in the steam era, and a train horn as you find on a modern diesel or electric– whistles use a resonant chamber, while a horn uses a diaphragm to create the vibrations we call sound. By using the movable chamber caps to change the resonance, you can go from a Santa Fe 5 chime to a B&O 3 chime whistle by matching the measurements in the handy-dandy spreadsheet [TrainArtisan] provided– and doubling up on a couple of chimes.One excellent tip he gives in the video is to grease all the mating surfaces of the whistles with petroleum jelly, but it’s worth watching the whole thing on YouTube for both full build and tuning instructions, and to learn more than you ever thought you needed to know about whistles– he gets right into the physics of sound waves to explain exactly the limitations of trying to replicate the sound of brass steam whistles with plastic and compressed air. If you just want to hear a bunch of train whistles to see how well it works, jump to the ten minute mark. The disappointing part comes later, where he explains the airflow requirements: if you don’t have a compressor with a big tank, you can’t expect more than one short toot at a time.Interestingly enough it looks like most of the whistles we’ve written about are locomotive-adjacent, including [AeroKoi]’s earlier work, and a true cast-brass replica for the purists.