hmmmmm...well there's also Thomas Grills idelay which does interpolating delays [i spent one beautiful morning with that external pitchshifting tedious breakfast radio broadcasts, fun was had by all]. i'd imagine really complex and realistic doppler panning is impossible without lots of work as your man, matthew jones, said but if say, you only want to do an "oncoming train" [or indeed a departing train] type effect then perhaps you could put together an abstraction that goes before a vd~ or file player or synth that uses an expr [or something] which does a doppler effect equation and outputs the pitch and amplitude variations needed [you might also need a small amount of reverb or filtering to make it sound realistic rather than merely academic] but, well i haven't tried it [i think i shall though] so i don't know how it would sound...good luck with your continued attempts though...