The thread about Thomas the Tank Engine, and steam locomotive whistles reminded me of the Doppler Effect:

Doppler Effect, in physics, the apparent variation in frequency of any emitted wave, such as a wave of light or sound, as the source of the wave approaches or moves away, relative to an observer. The effect takes its name from the Austrian physicist Christian Johann Doppler, who first stated the physical principle in 1842. Doppler's principle explains why, if a source of sound of a constant pitch is moving toward an observer, the sound seems higher in pitch, whereas if the source is moving away it seems lower. This change in pitch can be heard by an observer listening to the whistle of an express train from a station platform or another train. The lines in the spectrum of a luminous body such as a star are similarly shifted toward the violet if the distance between the star and the earth is decreasing and toward the red if the distance is increasing. By measuring this shift, the relative motion of the earth and the star can be calculated (see Red Shift).



"Doppler Effect," Microsoft(R) Encarta(R) 98 Encyclopedia. (c) 1993-1997 Microsoft Corporation. All rights reserved.