Roughness and irregularities on the wheel and rail surfaces are a source of noise and vibration. Rail joints and squats on the rail cause a familiar clickety-clack sound as train wheels roll over them.

That rhythmic "clunk-clunk" sound is most commonly caused by rail joints. In older track systems, individual rails are bolted together with "fishplates," leaving a small gap to allow the steel to expand and contract with temperature changes. As each wheel passes over this gap, it creates an audible impact. In 2026, most major high-speed and mainline tracks have transitioned to Continuously Welded Rail (CWR), which eliminates these gaps and the resulting noise. However, you will still hear clunking when a train moves over switches (turnouts) or "frogs," where the wheels must bridge gaps in the rail to change tracks. Another cause can be "flat spots" on the wheels themselves; if a train’s brakes lock up and the wheels slide along the track, the friction wears a flat area into the circular wheel. Every time that flat spot hits the rail during rotation, it produces a persistent, heavy thumping or clunking sound that can be felt by passengers inside the carriage.