Railway Track Tolerances

Track Tolerances

The safety and comfort of travel depend primarily on track geometry and the standard at which it is maintained. In practice, it is not possible to obtain a flawless and perfect track; the parameters governing track geometry always show certain variations due to various reasons. Track tolerances may be defined as the limits of variability of various parameters pertaining to track geometry.

1 Track Parameters

Tolerances are generally laid down for the following track parameters.

Gauge variation This is measured as the deviation from the nominal gauge, which is 1676 mm for BG and 1000 mm for MG. The gauge is measured as the minimum distance between the running faces of two rails.

Unevenness This is measured in terms of the difference in longitudinal levels over a fixed base. Unevenness is generally measured over a base of a length of 3.5 m. It is measured separately for left and right rails.

Cross level difference This is measured in terms of the relative differences in the level of two rail tops measured at the same point. Cross level difference also includes the variations in superelevation.

Twist This is measured in terms of the change in cross levels per unit length of measurement. Twist is calculated after knowing the cross levels and the difference between two points over a fixed base of, say, 3.5 m and dividing the cross level difference by this base length. This is normally denoted as mm/m.

2 Safety Tolerances

Safety tolerances are the limits of variation beyond which the movement of traffic on the track becomes dangerous or unsafe. The kind of unsafe conditions that arise depend not only up on the condition of the track but also on the type of vehicle, its riding characteristics, and its standard of maintenance. The factors that govern these unsafe conditions are so variable and indeterminate that almost none of the railway systems in the world have laid down any safety tolerances. It is generally believed that possibly the track would have to deteriorate to a great extent for it to reach these unsafe limits.

3 Track Tolerances for Good Riding Quality

The limits of track tolerance prescribed in order to guide engineering officials regarding the suitability of the standards of track maintenance on BG tracks with a sanctioned speed of above 100 km/hr and up to 140 km/hr are presented in Table 18.4.

Table 18.4   Limits of track tolerances on IR

There are no special tolerance limits for cross level defects. The track should be maintained at a standard that is generally superior to that of main line tracks where unrestricted speeds of up to 100 km/hr are permitted.

The provisions and tolerances mentioned here and elsewhere in the chapter have been provided with a view to maintain track geometry so as to ensure a comfortable ride and not from the point of view of stability or safety. (Source: ACS No. 96 of 2004 issued under Railway Board letter no. 2004/CE-II/CS-I dated 22 July).