It has to do with the height and strength of a temperature inversion just above the ground. On clear, calm nights, it is cooler at the ground than higher up. When air warms with altitude a temperature inversion exists. Sound travels faster in warm air than it does in cool air.

Hearing the sound of a distant train is a high-fidelity phenomenon influenced by atmospheric acoustics and ambient noise levels. Sound travels much more efficiently in cold, dense air or when there is an "Inversion Layer" that reflects sound waves back toward the ground. This is why you may hear a train more clearly at night or in the early morning when the air is still and the high-fidelity "Noise Floor" of daytime traffic and human activity has dropped. Furthermore, moisture in the air (high humidity or fog) can actually help carry sound over longer distances. The high-fidelity "Doppler Effect"—the change in pitch as the train moves toward or away from you—also makes the sound more distinct to the human ear. If you only hear it occasionally, it may be due to the "High-Fidelity" wind direction carrying the sound toward your home, or a change in the train's schedule, such as a "High-Fidelity" heavy freight line that only runs on specific nights to avoid daytime passenger congestion.