In addition, the required precision of the grinding process must be defined in order to establish the acceptable tolerances and corresponding allowable deviations from the desired template shape. The tolerance can vary as a function of the horizontal position along the railhead width. A finer tolerance (smaller allowable deviation), for example, may be set near the gauge point of the high rail on a curve, because of the importance of that location in achieving the optimal rail shape and its associated improved wheel/rail contact surface. Likewise, where the accuracy of the wheel/rail contact surface is a lesser concern, the tolerance (deviation) may be increased. This tolerance can be considered as an acceptance envelope, and the need for additional profile grinding is determined based on the magnitude of the difference curve as compared to the acceptance envelope

Figure 2 shows a single tier acceptance envelope (0.25 mm or 0.010 inch) with the difference curve displayed graphically. Note that portions of the railhead are within the acceptance envelope, requiring no additional metal removal. Portions above this envelope require additional metal removal. The GQI is then calculated as the percentage of area that still needs to be removed (by grinding) to achieve the template, or target profile (3).

The scale of the GQI is 0 to 100, and reflects the degree to which the difference curve falls within the acceptance envelope. If the difference envelope across the entire railhead falls within the acceptance envelope, then the GQI is equal to 100. This represents a fully acceptable profile and no additional grinding is required. Conversely if the difference envelope across the entire railhead falls completely above the acceptance envelope then the GQI is equal to 0. This represents an unacceptable profile and significant additional grinding is required.

The GQI serves as a quality control tool by allowing comparison of a “before” index (in front of the rail grinder) with an “after” index (behind the grinder) to determine how effective the grinding pass was in achieving the desired profile shape. Since the analysis is performed separately for each rail, GQI values are calculated for both the left and right rails, again permitting quality control monitoring for each rail.