(dc) cool effect

i noticed yesterday morning while walking from the l'enfant plaza metro station to the nasa hq building (which takes me roughly under the train tracks) a very interesting effect. i was walking east along school street and heard the train coming from behind me. just as it passed me, it started blowing its whistle. since it was already past me, the whistle just kept blowing at a constant pitch as i walked. there are a lot of large reflective buildings around there. i didn't notice at first, but then suddenly realized that the train's echo was a higher pitch than its whistle. i spent the next few steps figuring that while the train was going away from me, it must have been going towards the buildings off of which the original waveform was bouncing to come back at me. everyone is familiar with the way a horn or whistle will shift down in pitch as it speeds past you, but how many times have you noticed a train whistle echo blue-pitch-shifted up from its source?

yes, i'm a nerd. but i thought this little doppler effect was cool.