As for populated areas, they are where there are usually people driving around and crossing the tracks. The train needs to blow it's horn very loud when it goes over a crossing to let people know that it's there so they don't drive in front of it and get squished.

Amtrak trains are often perceived as loud due to a combination of federal safety regulations, the mechanics of heavy rail travel, and the environment in which they operate. The most distinct "loud" sound is the locomotive horn, which is mandated by the Federal Railroad Administration (FRA) to be sounded in a specific sequence (long-long-short-long) when approaching public grade crossings. These horns must reach a decibel level between 96 and 110 dB to ensure they can be heard by motorists and pedestrians over engine noise and road traffic, preventing potentially fatal collisions. Beyond the horn, the trains generate significant "rolling noise" from the steel-on-steel contact between the wheels and the rails, which can be amplified if the track is older or has joints. Additionally, many Amtrak routes run through densely populated "corridors" where the echoes off buildings make the mechanical sounds of the diesel engines and cooling fans more noticeable. While some communities establish "Quiet Zones," the engineers are still required to use the horn if there is any safety hazard, such as a person or animal near the tracks.