Rail irregularities, corrugation and acoustic roughness: characteristics, significance and effects of reprofiling

Railway noise is excited by irregularities on the running surfaces of wheels and rails, with rails being more significant for most of the wavelength and frequency range of interest. Measurements are presented that demonstrate the significance of irregularities in the 100-1000 mm wavelength range (corresponding roughly to 25-250 Hz) on ground-borne noise, and show that a reprofiling specification to address reductions in low frequency noise should address these longer wavelengths. Differences are demonstrated between irregularities in the 6.3-2000 mm wavelength range on a dedicated high speed line, and on heavy haul, mixed traffic, metro and light rail systems. Although the sample size is small for the high speed and heavy haul systems, the lowest levels of irregularity were nevertheless found on the high speed line. Heavy haul systems also have low levels of irregularity, which are below the limit specified in the acoustic standard ISO 3095. Metro systems are distinguished by corrugation, higher levels of short wavelength roughness corresponding perhaps to roughening from consistently high traction and braking, and significant differences between high and low rails in some curves. In one case shown here the high rail is amongst the smoothest in an entire sample of dozens of rails whereas the corresponding low rail is by far the most heavily corrugated and irregular. Light rail systems have relatively high levels of irregularity, particularly at short wavelengths. This may result from sanding to enhance adhesion. Reprofiling in general reduces irregularities below the level stated in ISO 3095, except at wavelengths of less than 30 mm for which irregularities are consistently greater after reprofiling than before. Reprofiling is particularly effective in reducing irregularities in the 30-500 mm wavelength range, in which irregularities are typically reduced by 10 dB but in some cases by 20-30 dB. Irregularities to the ISO 3095 limit spectrum exist immediately after some forms of grinding. The best results are obtained from reprofiling that is undertaken to an appropriate specification that is conscientiously monitored.