Derailment and its Causes
As mentioned earlier, out of all the various types of railway accidents, derailments are the most common. In the case of a derailment, an engine or a wagon skids off the railway track due to one or more relevant factors crossing their safety limits. Derailment may generally be of two types.
Sudden derailments due to wheel jumping off the rails This type of derailment is not characterized by a mounting mark on the rails but by a drop mark on the sleepers. It occurs when the derailing forces are high enough to force the wheel to suddenly jump off the rails. The probable causes of this type of derailment are listed below.
(a) Excessive speeds, particularly at a curve or turnout
(b) Sudden shifting of load
(c) Improperly loaded vehicles
(d) Sudden variation in drawbar forces caused due to braking or acceleration or improper train operation
(e) Resonant rolling, rising, or hunting
(f) Broken wheels or axles
(g) Failure of vehicle components
(h) Presence of some obstruction on the track
(i) Failure of track components. If both the wheels drop on the inside of the track, it may be a case of track distortion or gauge expansion.
Gradual derailment due to flange climbing This type of derailment is caused by the wheel flanges gradually climbing onto the rails, thus leaving mounting marks on the rail. Such a derailment occurs when the derailing forces overcome the stabilizing forces but are not high enough to cause a sudden derailment. In this case, the unstable condition of the track causes the wheels to climb onto the rail, ride over it, and finally slip and slide off it. It is difficult to establish the prima facie cause behind this kind of derailment, and detailed investigations need to be carried out so as to finally arrive at the exact cause for the same. The reason for such an accident may be the single or joint effect of the following factors.
(a) Track defects
(b) Vehicular defects
(c) Unfavourable operating techniques
All these causes are further elaborated upon in Table 22.3.
Table 22.3 Details of defects that lead to derailments
1 Derailments on Turnouts
A large number of derailments occur on turnouts because this is where vehicles change the track they are on. The reasons for these derailments are generally as follows.
(a) Improper manipulation of points
(b) Gaping points
(c) Track defects
(d) Worn out wheel flanges
(e) High wing rail and loose crossing bolts
2 Nadal's Formula for Derailment Analysis
At the time of derailment, when the wheel flange is in the process of climbing onto the rail, certain forces act at the part of contact between the rail and the wheel, which lie at positive angularity to each other as shown in Fig. 22.1.
Nadal's formula for safety against derailment is
where Y is the flange force, Q is the instantaneous wheel load, R is the normal reaction of the rail, m R is the frictional force acting upwards, m is the coefficient of friction between the wheel flange and the rail, and b is the flange angle. Y/Q is known as the derailment coefficient. Nadal's formula provides an important criterion for the assessment of the stability of rolling stock. It also has practical use in the investigation of derailments. Based on Nadal's formula, the factors that contribute to derailments, whether due to track defects, vehicular defects, or unfavourable operational techniques, are as follows.
(a) Increase of flange force Y
(b) Decrease of instantaneous wheel load Q
(c) Increase of coefficient of friction between the wheel flange and the rail (m) . This is normally the consequence of a rusted rail, a newly turned wheel standing on the rail, sharp flanges, etc.
(d) Increase in positive angle of attack condition ( b )
(e) Increase in positive eccentricity. This increases primarily due to the slope of the wheel flange becoming steeper.
(f) Persistent angular running of the axle.