I just have to "blow the horn" of Rapido Trains, an MRH sponsor.  Their detailed models have set the bar high for a lot of manufacturers and their latest FP9 continues to set the standards for what a plastic loco can be.  One of these innovations (in conjunction with Soundtraxx), is to add the doppler effect to their array of sounds.  Take a listen:

While its not a specific manipulation, it does sound kind of cool.  It will only work as a "long, long, short, long" horn as it is a sample to use at grade crossings.