Most Helpful Guy
It is because mornings often have a temperature inversion.
Normally, the air temperature gets cooler the higher you go. Due to radiative cooling, the ground at night can often cool off faster than the air, so the air near the ground is cooler than higher up.
The speed of sound through air depends on the temperature. With the change of temperature going up in altitude, sound waves are refracted (bent) back down to the surface, similar to how light bends as it enters and leaves a lens due to the different speeds of light through air and glass or plastic.
The effect is that sound which would have escaped high into the air is bent back to earth and can travel long distances along the surface. And that is how you come to hear it.
You can also sometimes get sound reflections off of clouds and have the same effect as a temperature inversion.
Most Helpful Girl
Lots of freight trains run at night, when there are fewer passenger trains.
Or maybe it's a ghost train!