Relays! I'm ashamed I didn't suggest those first. One of life's most precious moments was touring the MIT Tech Model Railroad Club during the last year of thei

Relays! I'm ashamed I didn't suggest those first. One of life's most precious moments was touring the MIT Tech Model Railroad Club during the last year of their old switching system's operation. It was built around some old relay-based telephone switch the size of a couple of refrigerators, and the whole thing made these awesome clacking noises. It also emitted this constant, periodic metallic click, like a metronome, which I was told was the clock.