Suppose there is a steam locomotive in the old west traveling at 110 km/h. Assuming the speed of sound as 343 m/s, that means that the Doppler shift for an approaching train would be about 1.098, while for a receding train would be about 0.918 ... dividing the two to find the overall frequency difference gives about 1.196 .
This is very nearly 1.2, or 6/5, the interval of a justly-tempered minor third, which is scientifically known to be the saddest musical interval. Even if it's moving at an angle, or slower due to inferior-quality scrap rail, an equally-tempered minor third is tighter, about 1.189 . This is my scientific justification as to why the wail of a steam train is often described as "mournful" or a "wail".
However, I am suspicious as to why the steam train in the old west is using metric units. It may have been infiltrated by the French, or possibly Doc Brown.