Squeal is generated from lateral slip between wheel and low rail. Further, it is understood that squeal occurrence is dependent on the friction behaviour in combination with the size and direction of the forces acting in the contact patch; more specifically, the presence of the so-called 'stick?slip loop.

The high-pitched squeal heard when a train rounds a sharp curve is primarily caused by a physical phenomenon called "Stick-Slip" friction. Unlike a car, whose wheels can turn at different speeds using a differential, a train's wheels are fixed to a solid axle and must rotate at the exact same rate. When entering a curve, the outer wheel has a longer distance to travel than the inner wheel. Because they are locked together, one or both wheels must "slip" or "slide" across the top of the rail to compensate for the distance difference. This rapid sticking and sliding creates intense vibrations in the wheel, which acts like a giant circular bell, radiating the high-frequency "squeal" into the air. In 2026, many modern rail systems use "friction modifiers" or automated track lubricators to reduce this noise. Another source is Flange Rubbing, where the vertical "lip" on the inside of the wheel physically grinds against the side of the rail, a sound that is often lower in pitch but equally grating and indicative of wear on the track.