Transport for London (TfL) is exploring new ideas to reduce screeching noise from trains inside tube tunnels. Most of the noise comes from wear and tear on the top of the rail where the surface becomes affected by a type of track wear caused by repeated acceleration and braking, called rail corrugation. Once this process has started, it will worsen exponentially as time progresses.
Noise on the London Underground has worsened in recent years thanks to the introduction of automatic train operation, which can allow trains to run more reliably and closer together but also means there is more acceleration and braking by the automatic system, and that increases rail corrugation on the tracks. There is less corrugation on manually driven trains with more speed variation (Bakerloo and Piccadilly lines), as this map shows.