Why are underground trains noisy?

Underground trains produce noise primarily due to several factors:

Wheel and rail interaction: The contact between the train's steel wheels and the steel rails creates friction and generates noise. As the wheels roll, they create a 'clickety-clack' sound, which is amplified in the enclosed underground tunnels.

Track irregularities: The underground rail tracks may have unevenness, joints, or switches that can cause the wheels to impact or vibrate, producing a rumbling or banging noise.

Brakes and acceleration: When trains brake or accelerate, the friction between the wheels and rails increases, generating a screeching or grinding sound.

Tunnel acoustics: Underground tunnels act as sound amplifiers due to their enclosed structure. The reverberation of sound waves off the tunnel walls and ceiling magnifies the noise produced by the train.

Squealing of wheels: When trains negotiate tight curves, the wheels may squeal due to the increased friction between the wheels and the rails.

Ventilation and cooling systems: Underground train systems often have extensive ventilation and cooling systems to provide fresh air and regulate temperature. The noise from fans, pumps, and compressors associated with these systems can add to the overall noise levels.

Electrical systems: Underground trains use electrical traction motors and power supply systems, which can generate humming or buzzing sounds.

Construction and maintenance: Ongoing construction or maintenance activities in the tunnels can also contribute to temporary noise levels.

To mitigate these noise issues, underground train systems often employ various measures such as track maintenance, wheel dampening, sound-absorbing materials in the tunnels, and noise-reduction technologies in train designs and operation.