Why do train cars squeal?

The most common cause of train squealing is the friction between brake pads and wheels or brake discs during deceleration. When brakes engage, metal-on-metal contact generates heat and vibrations that translate into loud squeals. This noise intensifies during emergency stops or when brakes are worn, as degraded materials struggle to maintain smooth contact. High-speed trains often use dynamic braking systems, which can amplify screeching sounds due to rapid energy dissipation.

Wheel and Rail Interaction

Squealing can also arise from the interaction between train wheels and the tracks. As trains navigate curves, the wheel flanges press against the rails, creating friction and vibrations. Additionally, uneven wheel surfaces—such as wheel flats (deformed spots from sudden braking)—can scrape against rails, producing high-pitched noises. Track irregularities, like uneven joints or worn grooves, further disrupt smooth movement, leading to persistent squealing.

Worn or Faulty Components

Mechanical wear and tear contributes significantly to train noise. Worn bearings, gears, or suspension parts can vibrate at specific frequencies, generating squeals. Poorly aligned axles or loose components may also rub against adjacent parts, amplifying vibrations. Material factors play a role too: harder metals used in wheels and rails produce sharper noises when they grind together, especially under stress from heavy loads or sudden acceleration/deceleration.

Lubrication deficiencies in moving parts like pivot points or couplings can also cause metal parts to scrape, emitting squealing sounds. These issues are often addressed through regular maintenance, including lubrication, part replacements, and track realignment to reduce friction-induced noise.

What makes the choo choo sound on a train?

Steam Exhaust and Engine Valves

The iconic "choo choo" sound originates from the rhythmic release of steam in steam locomotives. As the engine operates, high-pressure steam is directed into pistons to drive the wheels. After each power stroke, excess steam is expelled through exhaust ports, creating a sharp, explosive burst. This process repeats with each piston cycle, producing the distinctive "choo-choo" rhythm. The sound’s intensity depends on the engine’s speed and the timing of the regulator valve, which controls steam flow and exhaust release. The repeated popping noise occurs as steam escapes through the cylinder valves and blast pipes, synchronized with the locomotive’s motion.

Steam Whistle and Stack Design

Another contributor to the classic "choo choo" is the steam whistle, which uses pressurized steam to produce loud blasts. While the whistle’s sound is distinct from the engine’s exhaust, it’s often associated with the overall locomotive noise. Additionally, the exhaust stack (or chimney) design plays a role. The stack’s shape and spark arrestor can amplify or modulate the sound of escaping steam, enhancing the recognizable "choo" effect. Together, these components create the signature auditory experience of early trains.

Modern Trains and the Evolution of Sound

Modern diesel and electric locomotives lack steam engines, so their "choo choo" sound is absent. Diesel engines produce a deep, rumbling exhaust from internal combustion, while electric trains generate noise from traction motors and wheels on tracks. The classic "choo-choo" is specific to historical steam locomotives, where the mechanical expulsion of steam was a byproduct of power generation. Today, the sound persists mainly in nostalgic references and toy trains, but actual trains rely on horns and mechanical hums instead.

What is the train noise called?

The distinct sound produced by trains is often referred to as a train horn or air horn, depending on the type of noise. These horns are designed to alert pedestrians, drivers, and other trains to the train’s presence, especially at crossings or in populated areas. Modern trains typically use electronic horns that generate a loud, deep blast, while older locomotives might still use traditional steam whistles, which produce a high-pitched, whistling sound. The primary purpose of these noises is to ensure safety by signaling the train’s approach and preventing accidents.

Another common train noise is the screeching or squealing sound heard when brakes are applied. This is often called wheel squeal or brake squeal, caused by friction between the train’s wheels and the tracks. It occurs during sudden stops or emergency braking, though it can also happen during regular deceleration. This noise is both a byproduct of mechanical braking systems and a warning to nearby individuals of the train’s movement.

Trains may also emit bell-like sounds from onboard bells, though these are less common in modern rail systems. Additionally, some trains use voice announcements through external speakers to communicate specific warnings, such as “Train approaching!” These verbal alerts are part of enhanced safety protocols in urban or high-traffic areas. The choice of noise type depends on the train’s design, location, and regulatory requirements.

Why do trains make that clackety clack noise?

The rhythmic "clackety clack" sound of trains is primarily caused by the interaction between the train’s wheels and the tracks. As the metal wheels roll over the rails, they pass over gaps between individual rails called rail joints. These joints, spaced at regular intervals, create a slight slack or separation. When the wheels hit these gaps, they momentarily jump slightly, causing a percussive click. This noise is amplified by the speed and weight of the train, making it audible even from a distance.

How wheel design contributes to the sound

The flanged wheels of trains also play a role. The flange—the raised edge on the inner side of the wheel—helps keep the train on the tracks by pressing against the rail. As the wheel rotates, the flange periodically contacts the rail, creating vibrations. These vibrations turn into sound waves, producing the familiar clacking noise. Factors like the surface texture of the rails and the material composition of the wheels can alter the pitch and intensity of the sound.

Maintenance and track conditions

Worn or uneven tracks can intensify the clacking noise. Over time, constant friction between wheels and rails wears down the metal surfaces, creating irregularities. Additionally, loose bolts, misaligned joints, or debris in the track bed may cause increased vibrations. Modern railways often use continuous welded rails to reduce gaps and minimize noise, but many older lines still rely on segmented tracks, maintaining the classic clackety clack sound.

Speed and physics of vibration

The faster a train travels, the more frequently the wheels hit the rail joints, increasing the rate of the clacking sound. Physics principles like resonance and acoustic amplification also come into play. The hollow structure of train wheels and the rigid rail system act as a resonating chamber, projecting the noise outward. This effect is especially noticeable in open areas or when tracks are elevated, such as in viaducts or bridges.