Why does the cell phone signal disappear when the train passes through the tunnel?

General mobile phone signals (except maritime satellite mobile phones) are sent to mobile phones through encrypted digital electromagnetic waves of various ground base stations. With limited distance and transmitting power, this signal will be attenuated by various mountains and buildings, especially in mountainous areas and complex buildings. In complex environment, mobile companies generally adopt the method of adding base stations or gain amplification points to realize signal relay transmission. However, these signal amplification and gain devices are rarely installed in ordinary train tunnels. There is no signal in the long tunnel because of the blocking of the mountain.