I assume that the pitch changed depending on the loco's speed?
Is there a film or sound recording of the sound out there? If yes, both ESU and ZIMO (and also QSI) decoders are programmable with any sound you can imagine. If you had a sound source, you could tackle creating your own S2 sound project (or find someone experienced with creating sound projects to do that for you). Then upload the sound project to the decoder, and you will have one-of-a-kind model with accurate sound.