In the old days, the train signs made a mechanical noise as the cities, tracks, and times changed. Just like brake lights on the car ahead of us alert our brains to stop, the sound of the flipping digits told our brains to pay attention, and then to hustle. Alas, the old-fashioned boards at North and South stations in Boston were replaced by electronic signs, but they still can click, thanks to computer-generated recordings. If you want the real thing, though, you can get it at the much smaller rail station at Route 128 in Westwood, where the sign does its click-click-click as it cycles through the day.