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.

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.

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.

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

Parameter

Value

Remarks

Alignment defects

5 mm for a straight track

Up to 10 mm at isolated

(versine measured over

locations

7.5 m under floating

Curve 5 mm

Up to 7 mm at isolated

conditions)

Total change of versine from chord to chord

locations

<10 mm

Twist

2 mm/m for straight and

Up to 3 mm/m at any

(measured on a base of

curved track

isolated location

3.5 m)

1 mm/m for the transition

Up to 2.1 mm/m for

of curves

isolated locations

Gauge variation for

6 on a straight track

-3 to +6 on a straight MG

BG (mm)

-6 to +15 mm on curves

track

-3 to +15 mm on curves

with radius of 350 m or

with a radius 290 m or

more

more

Up to +20 mm on curves

Up to 20 mm on curves

with radius less than 350 m

with radius less than 290 m

Unevenness of rail joint

General: 10 mm

15 mm for isolated

depressions

(measured on a chord of 3.5 m)

locations

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).

Summary

The track should be maintained properly to enable trains to run safely at maximum permissible speeds. Through packing, overhauling, and picking of slacks are the three main operations of maintenance in one calendar year. The money and labour to be spent on track maintenance should be optimally utilized to keep the track in good working condition. There are various railway officials who are assigned specific duties in regard to track maintenance. Also, there are limits assigned for the variability in the different parameters that affect track geometry, known as track tolerances.

Review Questions

1. Categorize and briefly describe the various duties of a permanent way inspector.

2. Explain the system of annual maintenance of rail tracks on straight portions, including the several operations involved therein. Explain these operations in detail?

3. The following defects were recorded upon inspection of a railway track.

(a) Battered ends of rail joints

(b) Angular cracking of rails

(c) Excessive closing and opening of spaces at rail joints Give reasons for these defects and suggest any remedial measures.

4. Describe the procedure involved in the annual through maintenance of a track commonly known as through packing.

5. What is the need for the proper maintenance of a track? Discuss the various methods that ensure that a track is well maintained.

6. What is meant by through packing? Describe the various steps involved in this procedure. What is the programme of annual track maintenance followed on Indian Railways?

7. Differentiate between the following.

(a) Hogged joint and high joint

(b) Lifting and lowering of track

(c) Insulated joint and fish-plated joint

(d) Defective sleeper and centre-bound sleeper

8. What do you understand by deep screening of ballast? Describe the procedure.

9. What are the duties of an assistant engineer on Indian Railways? What is the schedule of his or her inspection?

10. Write short notes on the following.

(a) Gang strength

(b) Blowing and pumping joint

(c) Tools of a permanent way gang

(d) Maintenance of track in track-circuited areas.

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