At night, when things are quiet, the noise can carry long distances. It is quieter at night therefore you can hear noises from further away than during the day when other noises “drown them out”.
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The reason that trains honk their horns so much at night is because it's dark and the trains aren't so easy to see. Even though the lights are on, we sometimes can't see them coming, especially around the many blind curves near or ahead of the train station.
Have you ever thought why trains blow their horns so often and so loudly? The truth is that they have to. And the main reason for that is safety. Locomotive engineers are required to honk every now and then, which is written down in the regulations called the Final Rule on the Use of Locomotive Horns.
Under the Train Horn Rule (49 CFR Part 222, issued on August 17, 2006), locomotive engineers must begin to sound train horns at least 15 seconds, and no more than 20 seconds, in advance of all public grade crossings.
The blast of the train whistle under temperature inversion conditions can to bend upward, traveling quickly through the warm air high above the ground before spreading downward through the cool air closer to the ground and into your ear through an open bedroom window.
Today, the only whistle signals you're likely to hear regularly are the grade-crossing warning (which is also often used to warn employees or others on the tracks); two (or three) shorts to indicate the engineer has received a signal to start the train forward (or backward); and one long blast when a train is ...
“People assume that trains are loud, but that assumption is based on the fact that when a train's gone past them in a station it's noisy. That's because the noise a train makes is mainly projected to either side. When trains are moving directly towards you they are barely audible–until it's too late.”
When air warms with altitude a temperature inversion exists.Sound travels faster in warm air than it does in cool air. This means the sound of a train horn will bend downward when it passes through an inversion causing sound waves to propagate farther than normal.
The horn alerts people that a train is approaching a railroad crossing. It can also be used to warn animals or trespassers in our right-of-way along a section of track. Many people don't realize that federal and state regulations require us to sound the horn whenever we approach any crossing.