Train Doppler

Puzzle: You’re traveling on a train and another train passes you in the opposite direction. As it does so, it honks its horn and you hear the drop in pitch of the horn from the Doppler effect. You estimate that the drop in pitch is about a minor-third interval. (A minor third is the difference between the note middle-C and the E-flat above it. You can guess this by singing a scale starting at the lower note until you hit the higher note.) How fast are the trains going? Assume both trains are going at the same speed.

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