Why is it so hot in the train? How to solve it?

The train is a tin car. In the process of high-speed operation, the population density is too high, the air is not circulating, the air permeability of the window is not good, people are inside, and the weather temperature is relatively high, which easily leads to very hot inside the carriage. The solutions can be as follows: first, when the temperature is high, turn on the air conditioner moderately to adjust the temperature; Secondly, the window of the carriage is open, so that the air inside can convection well. Third, turning on the electric fan in the carriage can also solve the problem of moderate heat.