The New Zealand Railways Magazine, Volume 3, Issue 5 (September 1, 1928)
Derailments and Their Causes
The question of derailments and the determination of their causes, is one of the most important that comes within the preview of railwaymen. In the following article Mr. W. C. Bishop, of the South African Railways, throws much interesting light on the subject, and urges railwaymen to ascertain all that can be known about it.
Railway literature is curiously deficient in regard to railway accidents and their causes. Beyond short notes and articles scattered through the technical Press there is little to help the student. How would a young man equip himself with knowledge so that he could take his place if called to serve upon a Board of Inquiry dealing with a derailment? What study and reading would such a young man have done? Would he just blindly depend upon his practical knowledge? Not only is study and reading upon all branches of Railway operation necessary in the circumstances indicated, but an earnest attempt should be made to develop the analytical and judicial qualities of the mind, qualities present in most men, but dormant in nearly all, if the individual would wish to be fair and impartial and reasonably correct in his judgments.
What shall we describe as the “Working Tools” of an Inquiry Board officer?
It goes without saying that he has a good all round practical experience in his own branch, but this is not sufficient. As, under Transportation all officers receive a unique opportunity of gaining knowledge of all branches, he should spare no effort to gain a fair grasp of all branches so as to be in a position when necessary to put in a minority report, and not be too dependent upon other members.
He must, as I have said before, earnestly endeavour to cultivate the analytical and judicial qualities of the mind and above all must be fair and honourable. Then he should have a good grasp of the books of the “Railway Law,” i.e., General Appendix, the General Regulations and the General Train Regulations. Having arrived at this point he will, by constantly reading technical journals and studying out problems, gradually equip himself so that he brings a well balanced mind to the service of the Board.
Judge Graham (a South Africa Judge) once said that the Railway experts are too fond of setting up fantastic theories and ignoring plain facts. The more fantastic the theories endeavouring to explain the seemingly unexplainable, the better they were pleased. I think (after being engaged on inquiry work for over 22 years), we are all at times a little prone to this fault. The difficulty is that text books do not exist from which one can see the road of investigation pointed out clearly.
Students in South Africa are fortunate, however, in that an earnest attempt to publish matter on Derailments and their Causes was made by J. D. Shannon and A. J. Beaton in 1913 and 1915 respectively.
I invariably ask junior officers if they have read Shannon on “Derailments and their Causes” and Beaton on “Speed of Trains on Narrow Gauge Railways,” and invariably get a blank stare with an admission that they have never seen these works. These two publications are unique, and no railway officer can afford to be without them.
It may not be outside the range of speculation that in the future inspectors and officers may be required to have a good knowledge of the principles that cause derailments, and I cannot do better than advise all young men to study Shannon and Beaton very thoroughly.
Enquiries into derailments and their causes have, as their primary object, the elimination of the causes and the correcting, if possible, of errors and mistakes, so that danger to life and limb and property may be avoided in the future. The verdict of “Cause Unknown” is the most unsatisfactory that can be given; it does not assist the administration to guard against a recurrence.
Accidents may be classified as generally falling under one or more of the following heads:—
1. Human Element.
4. Rolling Stock and Engines.
The task before an enquiry board dealing with a derailment is not an easy one because:page 34
(1) An immediate inspection by a Board is not always possible.
(2) In serious accidents the evidence is almost always obscure (if not altogether destroyed) as the result of the accident and the track being torn up.
In determining the cause of accidents remember the Law of Average and the Theory of Probabilities.
Constants in railway operation are:—
(1) Millions of train miles run annually and the rarity of serious accidents.
(2) Varying speeds, from slow to high, with frequent change of conditions, i.e., grade and curvature.
(3) Varying types of vehicles.
(4) Loaded and unloaded vehicles, producting variation in heights of buffers.
As a cause for derailment one should be very careful in hastily adopting this theory, for:
(1) Experienced railwaymen are invariably unable to judge speed correctly.
(2) The capacity for speed of engines is not properly understood by most people.
Beaton says that the relation of speed to the condition of the track cannot be reduced to a formula. Also that elimination of sharp curvature and steep gradients will enable considerable increases in speeds to be made, and that, if it were practicable to increase the radius of all curves to 25 chains no speed restriction would be necessary for curvature. Beaton even goes so far as to say—and I am fully in agreement with him—that lateral vibration decreases with high speeds, while the vertical vibration due to “slacks” or low joints has a much less magnitude of range at high than at low speeds.
The tendency of a body in motion is to follow a straight line. Hence, on a straight track the tendency of a travelling vehicle is to remain in a position central with the track. Under ideal conditions, therefore, there are no reasons why an unlimited speed should not be permitted, on any gauge, on a perfectly maintained straight track with well-balanced stock (See Beaton, on “Speed of Trains”). “On a straight track with easy curvature no speed restriction would seem to be necessary,” says Shannon in “Derailments and their Causes.”
Under normal conditions there will be no pressure between the wheel flanges and the rail.
The centre of gravity of the train, and of each truck of the train, must move in a straight line on a straight track of uniform grade. In other words, the train must move in a straight line parallel with the track.
The irregularities in supposedly straight track cause trucks to oscillate, pitch and twist.
Oscillation is started by inequalities in the track. It is reduced, or absorbed, by the varying compressions on the springs, which first stop the rocking and then throw it back in the opposite direction. When the spring capacity is insufficient or defective, the oscillation will increase progressively until the wheels on one side are lifted off the rails. Ordinarily such oscillations are dissipated through the springs, but even when the springs are functioning, if there is a succession of cross level variation, the oscillation may become violent. When the load is at the point of maximum oscillation there is a much larger proportion of load on one side than on the other, and at that particular moment the wheels on one side carry much more weight than those on the other. As acceleration varies with a given force inversely as the weight and directly as the time, it follows that when a vehicle is at the point of maximum oscillation (with springs fully compressed and with the vertical reaction through the wheels on one side suddenly increased while being correspondingly decreased on the other) the acceleration of the lighter side is suddenly increased and that of the heavy side is suddenly decreased. The result is that the truck is suddenly and violently slewed, and derailment follows.
This will explain why trucks go off the road on straight track—a fact which has puzzled many railway men. Remember the point of derailment is not necessarily the cause. A track irregularity away back may be (and invariably is) the starting moment of oscillation.
If these occur on straights it is usually due to a variation of moving weights (water sloshing page 35 from side to side, due to the baffle plates inside not keeping a dead balance of water), or to track irregularities. In such circumstances an oscillation is set up as explained in the previous paragraph. Tenders equipped with side bearings, having small clearances on both bogies, are liable to be derailed when they pass over a track with a low place in one rail. This is true even on tangents, as the surge of the water in the tank will probably cause the flanges of the front bogie to ride the rail at the low places.
Theory of Bunching.
If we take Beaton on “Speed of Trains” and Shannon on “Derailments and their Causes,” we find that speed is not such an important factor in derailments as is generally supposed. Assuming that the track is in good order and that the permissible speed is not exceeded speed can safely be eliminated as a factor in most derailments.
Change of Grade.
This cannot produce a derailment. As all vehicles travel in the same centre line one can dispose of such a theory. The Theory of Probability denies such a possibility.
(To be concluded.)