Doppler loop ?

you're probably going too slowly.  pitch change is proportional to speed.  speed of sound about 750 MPH.  so if train going 75sMPH, you'll get a 10% pitch increase as it approaches and 10% pitch decrease as it departs.  10% pitch change is very easy to hear.  turn on bell and/or whistle which has a nice doppler effect.

no wonder as I do not go over 45 mph for a freight train as I do not want the train to fly off the track.

Even 45sMPH is a 6% increase coming and 6% decrease going.  Adjacent keys on a piano are also about 6% apart in pitch so I'd think you should hear something...