The distance at which a train can be heard varies significantly based on environmental factors, the type of train, and the terrain. Under ideal conditions, such as a cold, clear night with low ambient noise, the low-frequency rumble of a heavy freight train can travel up to 10 to 15 miles (16–24 km) through the ground and air. This is because low-frequency sound waves have longer wavelengths that bypass obstacles more effectively. Atmospheric conditions play a major role; "temperature inversions," where warm air sits above cold air, can refract sound waves back toward the ground, allowing the noise to "skip" across the landscape for vast distances. Conversely, in a dense urban environment with high humidity or heavy foliage, the audible range might drop to less than a mile. In 2026, modern high-speed electric trains are much quieter than older diesel locomotives, often becoming audible only within a few miles, which has necessitated the use of high-decibel air horns that can be heard from roughly 1 to 2 miles away to ensure safety at rail crossings.