Derailment of rolling stock is defined as a wheel or set of wheels leaving the due place from the rail top surfaced.The derailment is serious occurrences and may also cause loss of human life besides the loss of Railway property. They also result in the heavy interruption to through traffic of trains leading to substantial loss of railway revenue. Therefore all efforts should be made to avoid derailments.
A derailment may be sudden or gradual due to a failure of one or more of the following :
- Operational factors
- Rolling Stock
- S & T
There are two broad categories of derailment:
- Sudden Derailment – Instant dismounting of a wheel from a rail.
- Gradual Derailment – Gradual climbing of a flanges on the rail.
The possible causes for a sudden derailment are:
- Sudden shifting of load
- Improperly loaded vehicle
- Excessive speed on curve or turn out
- Sudden variation in draw-bar forces induced due to improper train operations (sudden braking or acceleration)
- Broken wheels/springs or suspension gear components
- Failure of track or vehicle component
- Obstruction on track
The factors contributing towards oscillations and resulting in off-loading and lifting of an individual wheel under running conditions are:
- Unequal spring characteristics
- Vertical irregularities of track
- Uneven loading of wagon
- Axle Load Variations during run
- Dynamic Aspects
If the complete spring collapses, there is a serious danger of derailments the vehicle is considered to be unevenly loaded when the center of gravity of the load is not on the same vertical axis as that of a center of the vehicle.
The following are the major operational features which are significant in a derailment:
The speed of train plays a vital role in derailments. If speed is in excess of permissible speed, lateral forces on the flanges increase and the formation may not be able to resist this increased force thereby resulting in either flanges mounting or jumping off from the rail table.
Irregular or excessive loading may lead to derailment of a vehicle as the wheel may float due to off-loading.
Empty stock marshaled in between two loaded wagons forces the empty stock, especially the four-wheeler units, to jump off under the impact of drawbar forces and derail.
Manipulation of Point:
Due to manipulation of a point under movement, derailment occurs. In such cases, the leading wheel or trolley will travel on one side of the track and the trailing trolley will travel on the other track. This happens on points in facing direction only.
Operating Staff Failure:
Following problems may also result in a derailment:
- Points not being properly set and locked during shunting operation at the station.
- Operation of a point under wheel movement.
- Undetected obstruction between the toe of a switch and stock rail.
- Loose couplings.
Improper Train Operation by the Driver:
The sudden application of brakes cause bunching and Offloading of light loaded wheels. This may also result in derailment.