The RCA was originally developed for rail reprofiling work, to provide equipment that could be mounted on a grinding train to show where reprofiling was required and when work had been done satisfactorily. The accuracy required of such equipment was extremely demanding. Residual irregularities at the shortest wavelengths of interest for reprofiling are limited to a few microns, which requires that measuring equipment is able to differentiate between rail that has been satisfactorily and unsatisfactorily reprofiled to these tiny levels of residual irregularity. No equipment that was available at the time could do this and it is questionable whether any train-based equipment that is now available, other than the RCA, can do this now.
The requirements of such equipment are not only that it is extremely accurate, but also that it function satisfactorily during the reprofiling operation, that it be robust and simple to use, and that it operate throughout the full speed and wavelength range of interest in reprofiling. The environment in which this equipment works, with debris from grinding or milling, is itself a test of any system on a reprofiling train let alone an instrument that must routinely measure irregularities that whose height is a small fraction of the thickness of a human hair.
RCA measurements of a 300m grinding site before and after grinding. In the 30-100mm wavelength range shown, irregularities have been brought well within those allowed for Class 1 reprofiling in EN13231-3:2012: a level of +/-0.010mm is exceeded over 3% of the site on one rail and 0% on the other rail.
RCA measurements taken on a different 300m site before and after grinding. The percentage exceedence of the 0.004mm level in the 30-100mm wavelength range has been reduced from 46 to 0% on the left rail and from 37% to 0% on the right rail.