Following result is obtained by our Internet Research Algorithm:
A train horn can be heard up to 3 - 5 kilometers.
Also, it is quieter at night so you can hear the train horn from long distance.
Also in the night and in winter, the sound travels more distance :
In the daytime the ground air is warm, bends airwaves upwards so for certain distance, you ain't gonna hear any sound at all, but during the night, the wave bends downward, bounced off ground and then bends downward again.