The short answer

The train screeching noise is mainly caused by the design of train wheels, which results in the wheels sliding on the tracks along curves and certain parts grinding against the track.

The long answer

If you haven't yet had the pleasure of hearing a train screech or train squeal, take a listen to this video of a subway car rolling around a bend:

Trains make a screeching, squealing sound because of two main reasons:

Reason #1: Trains do not have differentials.

Differentials are a mechanical trick that helps wheels turn at different speeds, which is necessary when a vehicle is going around corners. Trains do not have differentials like cars do, which means that both wheels on an axle must rotate at the same speed.

When a train goes around a curve, the outer wheel has to travel a greater distance than the inner wheel. Without a differential, one or both of the wheels needs slip or slide along the track in order to make the turn. This slip-n-slide creates friction and causes train wheels to squeal and screech, especially on turns.

Side note: For more of an explanation on differentials, check out this  instructional video that was recorded in 1937 .

Reason #2: The train wheel flanges make contact with the rail.

A flange is a projecting rim on a train's wheels that is designed to keep the wheel on a rail. Because of all that slipping and sliding, it is especially important to have these components to prevent a train from falling off a track, arguably more inconvenient than the screeching sound.

When a train is making a turn, it causes the flange to make contact with the rail. And since it's steel rubbing against steel, this results in a lot of friction and – you guessed it – a lot of squealing.

In sum, trains make a screeching noise because of the wheels sliding on turns and parts of the train wheel grinding against the track.

Sources

Harnett, S. (2016, November 10). Why Are BART Trains So Loud? KQED. Retrieved April 21, 2023, from https://www.kqed.org/news/11030282/why-are-bart-trains-so-loud

Lubbock, R. (2017, May 1). Why The MBTA's Wheels Squeal — And How It's Trying To Dampen The Noise. WBUR News. Retrieved April 21, 2023, from https://www.wbur.org/news/2017/05/01/mbta-wheel-squeal

Toronto Transit Commission. (n.d.). Subway Noise and Vibration. TTC.ca. Retrieved April 21, 2023, from https://www.ttc.ca/service-advisories/construction-notices/Subway-Noise-and-Vibration