Train noise is vehicle noise created by trains. Noises may be heard inside the train and outside. Subway systems, light rail transit and freight trains can send loud train noise into neighborhoods. Organizations such as the World Health Organization and the U.S. Environmental Protection Agency have set guidelines for noise level decibel limits for rapid transit. Noise levels can be reduced by installing noise barriers next to the track. Traditional clickety-clack sounds occur as a result of gaps in the rail to allow for rail expansion. On most railways, the gaps are opposite each other and if the carriages are about the same length as the rails, an even clickety clack sound is generated. In the USA the rail joints are staggered, so not being opposite each other, a different and irregular sound is heard. Sources Several distinct sounds are created by various parts of the train, such as engines, traction motors, brakes, and the wheels turning on the rails. 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. Rail corrugation (a periodic wear pattern resembling corrugated materials) causes tonal noise and vibration; fine, short-wavelength corrugation is known as "roaring rails" due to its high-pitched sound, whereas coarse, long-wavelength corrugation can cause the ground and nearby buildings to vibrate. Rail roughness and corrugation are treated by grinding the rails. This reduces noise in problem areas although trains make a distinctive tonal sound on freshly-ground track due to the pattern on the rail left by the grinding process, which wears flat over time. Rail squeal is a sound caused by a train's wheels slipping under specific conditions, usually in sharp curves. Air displacement of a train in a tunnel can create noise from turbulence. Trains also employ horns, whistles, bells, and other noisemaking devices for both communication and warning. The engines in diesel locomotives and DMUs produce significant amounts of noise. Newer locomotives have become much quieter in recent years due to noise regulations being implemented by countries and regions. However, there are still times that a locomotive may develop a defect in its turbocharger, which produces a whine that can be heard for many miles. While this is rare, and usually the said locomotive gets put into the shop quickly, the noise has been compared