CREEP of Rails: Causes of Creep, Wave Theory, Change of Temperature Theory, Unbalanced Traffic Theory, Effect of creep, Measurement Of Creep, Remedial Steps against Creep, Track Maintenance against Excessive Creep, Measurement Of Creep, Track Lengths susceptible to Creep

CREEP of Rails

The longitudinal movement of rails in the track is called Creep. Creep on the rails vary from place to place. In some places it may be as much as few centimetres in a month whereas at some other location, it may be negligible.

Due to creeping of rails, the openings in the rail joints at the point of creep gets widened.

Causes of Creep

Many theories have been evolved to explain the reasons for creeping of rails in a track. Some are explained as below.

A. Wave Theory

According to this theory, a moving train sets up a wave motion in the resilient track portion of the rail. Immediately under the wheels of the train, the rails between adjacent sleepers get depressed or deflected somewhat due to vertical loading. The moving wheel pushes the elevated portion of the wave, which in turn forces the rail in the direction of traffic. The raised portion of the rail in front of the moving load is carried forward by the wheel and this helps in developing creep. The raised portion of the rail at the rear of the moving wheel then assumes its normal position.

Along with the moving wheels of the train, the depressions formed in the rail also moves with them. As soon as the loaded wheels pass over a depression, the portion of the rail springs back to its original level. In this way, the wave motion tends to move along the entire length forward with the moving train. Similarly the impact of several wheels at a rail joint also causes creep.

The pitch and depth of the wave in the track however depends upon the following factors:

Stiffness of the track

Spacing of the sleepers

Wheel base of the vehicles

Weight of the rails quantity and quality of the ballast

Standard of the maintenance of the track

Track modulus of the material. 

To reduce the creep, it should be ensured that the track possesses the following qualities:

Increased stiffness

Stable soil for formation

Angular ballast for better interlocking

B. Change of Temperature Theory

Development of creep rail may be caused by expansion/contraction of rails due to change in temperature. The change in temperature depends upon the range of temperature and the location of the track i.e.. whether the portion is exposed to sun or remains under shade. Creep is more rapid during hot summer as compared to cold winters

C. Unbalanced Traffic Theory

According to this theory, if heavy traffic moves in one direction and light traffic moves in opposite direction predominantly on a single line track, unequal creeping occurs between the two directions. The net effect of creep in the direction of movement of heavier traffic is more. In case of double line tracks , the movement of trains is more or less unidirectional and as such creep develops in both the tracks in the direction of movements of trains.

Besides the above theories, the creep may also develop due to the following factors:

1. Loose fittings of rails

2. Bad quality of sleepers

3. Bad alignment of curves

4. Too tight or too slack gauge

5. Gradient of the track

6. Life of the rails

7. Uneven spacing of the sleepers

8. High embankments.

9. Weight of the rails.

In fact, it is very difficult to predict the magnitude of creep. Some times, only one of the rails may also start creeping in a direction opposite to the other rail.

Effect of creep

Though there are several serious effects of creep, the most serious is buckling of the track. If proper attention is not paid, a buckled track may easily derail a train. The other harmful effects are as under:

The sleepers move out of square and position.

The gauge and alignment of the track gets disturbed.

The surface of the rails get disturbed considerably due to movement of sleepers from their packed loads. 

This causes uncomfortable riding.

Rail joints at the starting points of the creep get opened out of their permissible limit considerably. 

The stresses develop in the fish plates and bolts. 

This may sometimes break the fish bolts and the rail ends gets battered badly. 

At the point where creep ends, the rail joint gets jammed and thus prevents free expansion of the rail.

Points and crossings get disturbed and it becomes difficult to maintain the correct gauge and alignment. The movement of switches becomes difficult and even the interlocking arrangement is thrown out of gear.

The expansion gaps become either too big or too short.

Besides above defects, kinking of rails forcing the ballast and even smashing the fish plates and fish bolts are common

Measurement Of Creep

The measurements of creep in a track may be done in the field as explained below:

A. Fix two creep indication posts or unserviceable rail pieces on either side of the track in the formation such that their tops are about 25 mm above the rail level.

B. Fasten a string to the tops of the posts such that it passes freely below the rail bottom.

C. Make a chisel mark or a point mark on the side of bottom flange of the rail on either side of the track. Note down the time and date of marking .

D. After a particular interval of time, measure the distance between the chisel marks and the string. This distance is the required amount of creep which has developed during the given time interval.

Remedial Steps against Creep

The following precautions are required to be taken to prevent and reduce the creep of rails in the track :

A. Keep the sleepers always well packed with heavy angular ballast and provide wide shoulders as to resist the 

B. Keep all the rail joints levelled so that there are less chances of impact by the moving wheels .

C. Keep the fish bolts tight and oil them frequently so that the ends of the rails move freely under thermal stresses.

D. Provide anti-creepers or anchors on the top of sound sleepers. If the creep still continues, the number of anchors may be increased per rail panel.

Track Maintenance against Excessive Creep

The maximum permissible limit of creep is 150 mm. In case the creep exceeds this limit, the adjustments must be made as under :

A. Measure the expansion gaps and check against the total amount of gap in the length which should be equal to the standard expansion gap required at the given temperature multiplied by the number of joints in the rail panel.

B. Remove the keys and fish plates. Insert correct expansion liners before pulling the rail back with bars.

C. Tighten up the fish plates and insert key properly.

D. In case the total gap is more than the standard gap, the required closure rails may be used with a speed restriction of 30 Km/h till the gaps are removed. E. While adjusting the creep, the sleeper spacing should also be adjusted.

Track Lengths susceptible to Creep

The locations in track more susceptible to creep are :

Steel sleeper trackCST-9 sleeper track joining the wooden sleeper track.

Long stretches on gradients having dips

Approaches to major girder bridges or other stable structures.

Approaches to level crossings and points & crossings.

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