Why do trains whistle, especially when there are no obstacles ahead at night?

According to the relevant regulations of railway roads, when a locomotive passes through a crossing or turns, or there are pedestrians on the line, which involves driving safety, the locomotive driver should whistle. However, in the residential section where the railway is located, there are mountains and dense trees to the east of the railway, and the office areas and residential areas of relevant units to the west of the railway are mostly separated by walls, and few pedestrians cross the railway. There is no intersection in this railway. Although there is a small bend, the front line of sight is not affected. However, not all passing locomotives will whistle when they pass by, but some locomotives will whistle and some will not whistle, that is to say, this paragraph "can whistle or not." There are four strange phenomena in this section of the train whistle: first, some express trains don't whistle, while the local trains whistle. It should be said that in order to ensure safety, express trains should whistle to remind them, but the actual situation is that some express trains whizzed past without whistling, while the local train marched like an old cow pulling a cart and whistled at the top of its lungs for a long time. Second, whistling during the day, whistling at night, and even more unscrupulous at night. At night, the train is illuminated by headlights, which can remind pedestrians that no sound is allowed at all! But this is not the case. People who live by the railway tracks have a deep understanding. In the dead of night, when the train rings, it is difficult to fall asleep. The train rang again, upset; When the train rings three times, it rushes to the crown! Third, the roaring railway is ringing on both sides and low in front. The train's flute diverged in oblique thorn. Residents living along the railway will find the flute particularly harsh, while people facing the train or facing away from the train will rarely hear it or not. Therefore, the actual effect is to cause great interference to residents on both sides and little vibration to pedestrians in front. Fourth, the whistle does not cross. Whether there is an intersection ahead or not, whether there are pedestrians or not, it sounds right. A few locomotive drivers with some "qualities" will make a brief reminder with a slight whistle, while some drivers will whistle for a long time, "the sound is not amazing, and they will scold endlessly", no matter whether the residents along the line can't stand it.