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The New Zealand Railways Magazine, Volume 3, Issue 6 (October 1, 1928)

Derailments and Their Causes

page 36

Derailments and Their Causes

With this instalment Mr. Bishop concludes his interesting study of derailments and their causes. He takes the view and quotes the opinion of several authorities that the best protection against derailments, is good track conditions.

Sudden Application of Brakes

Shannon, in his work on Derailments, disposes most thoroughly of this theory as a cause of derailments. If thought were given to the thousands of times yearly, on most railways, that sudden applications of the brakes are made, without any ill effect resulting, this theory would be ruled out as a causative factor in derailments. But it is extraordinary how widespread is this theory of bunching (always coupled with a sudden application of the brakes), and how persistently it is advanced without a shred of supporting evidence. Shannon says, and says correctly, that when the vacuum brake has been applied throughout the train, if all the brakes are adjusted alike so that each vehicle is retarded by a force proportional to its weight, no vehicle will have any effect on any other in the train. Although such a nice adjustment cannot be expected in practice, yet under working condition when all brakes are on throughout the train, one vehicle will exert but little pull or push on another. When the brake is being applied it acts so quickly that even under the most favourable conditions the speed of the vehicle in front will not be retarded more than two miles per hour before the brake acts on the vehicle behind it. While this difference in speeds will, no doubt, result in a slight shock (if the train is running with loose couplings at the time of the application of the brakes) and slight tendency to derailment on a curve it is most unlikely to be the sole cause of a derailment; more severe shocks are caused by banking engines pushing a train in the rear in ordinary operations or when steam is shut off and an engine is allowed to “drift.”

Invercargill-Lyttelton Express at Palmerston Station, Otago.

Invercargill-Lyttelton Express at Palmerston Station, Otago.

What Shannon means, in simple words, is that the application of the brake throughout the train is to all intents and purposes instantaneous on each vehicle. Tests on 200 axles have given a complete application throughout in 3 1/2 seconds—this leaves little time for crowding.

The shocks caused by a sudden application of the engine brake must be greater than those caused by an application of the vacuum brake; yet, Shannon says, he could not trace one case in which the Board of Trade officers have ascribed a derailment to this cause. I have also examined many Board of Trade accident returns and cannot find “Brake Application” even mentioned in this connection.

Personally, from a fairly long experience, I am satisfied that the so called “bunching” derailments really result from oscillation brought about by track irregularities and defective springs; this, of course, apart from positive defects in tracks or trucks.

Derailments Caused by a Combination of Defects

This is the favourite finding of Boards of Enquiry, and it is the line of least resistance. Officers should resist the temptation to adjust their views to such a cause. Combinations of defects, of course, will occur, but in all cases there is an initial cause which should be established.

Shannon on Derailments.

Shannon Carefully examines the various causes of derailments, and classifies them thus:—

(a) Incorrect gauge.

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(b) Defects of surface, slacks and humps. (Slacks as a cause of derailments are regarded by Shannon as of such significance that he devotes, in the chapter thereon, 945 words to the subject.)

(c) Track out of alignment—“kick outs.”

(d) Unsuitable cant on curves.

(e) Curve worn rails.

(f) Broken rails.

(g) Points which may be open, or have a tendency to open.

(h) Defects in vehicles—defective springs, broken axles, worn or loose tyres.

(i) Difference in heights of coupled vehicles.

(j) Defects which may cause the frame of bogies to bind or catch on the main truck frame.

(k) Speed too high. (The only limitation of speed is due to sharp curvature, rough and weak track and unsuitable vehicles. (See previous reference to speed and “Beaton on Speed.”)

(l) Sudden application of brakes. (See previous reference to this under “Theory of Bunching”)

(m) Unequal distribution of the moveable load on the various wheels.

(n) Coupling of vehicles having different amount of overhang (shorts between bogies).

(o) Moving points when the train is passing over them.

(p) Propulsion of a heavy train round sharp curves.

Wellington-Petone Sea Wall after the Recent Storm.

Wellington-Petone Sea Wall after the Recent Storm.

Lubrication of Moving Parts of Vehicles.

There is no doubt that bogie centres and side bearers, horn cheeks, etc., should be well lubricated, but the claim that absence of lubrication does cause a derailment has not been proven, though, of course, lubrication does decrease the resistance to traction on curves and renders derailment less likely. It is principally on traversing curves that the benefit of lubricating the side bearers is felt.

Derailment of Trucks on Curves.

Arnold Stucki throws interesting light on this class of derailment, which apparently has no specific cause. The condition of a truck passing round a curve is divided into three distinct periods—entering, passing and leaving the curve.

On curves the outer rail is elevated so that, at a certain critical speed, the centrifugal force is balanced and the load on all the wheels is equalised. The speed under which this condition takes place will be called normal speed. When the truck is passing over the curve at normal speed the load is carried on the centre bearing and the bogies are free to swivel. If the speed is greater than normal the truck will tilt outward, and if it is less than the normal the car will tilt inward. In both of these cases the side bearers toward which the truck tilts assist the centre bearing to carry the weight of the truck body. This pressure on the side bearings grips the truck and prevents it from swivelling freely.

When the truck enters a curve, the front wheels being on a partially elevated track and the rear wheels on a level track, it is subjected to a severe twist, and if the framing is rigid the load is carried by two side bearings diagonally opposite each other. (Where vehicles such as passenger coaches have good springing and long bodies and are therefore more or less flexible, this does not result, but on short trucks and tenders where the distance between the wheel centres is comparatively short, it holds true.)

There is a tendency for wheels to climb the rail of curves, and the only reason that wheels do not climb in their effort to swivel the truck around the curve is because of the excessive vertical load which comes on them.

Stucki examines at great length the action of trucks passing through curves (a) entering, (b) passing, (c) leaving, and in conclusion arrives at:—

page 38

(1) That curves should be maintained true curves, and that there should be sweet transition from curve to curve and curve to straight, and that great attention should be paid to super-elevation.

(2) Under normal conditions there would be no pressure between wheel flanges and rails.

(3) That where track or truck imperfections exist there are, in every curve, at least six points where there is a danger of wheels climbing the rail.

(4) Therefore, to minimise these tendencies, it is advisable to make the side bearings as frictionless as possible.

(It will be noticed that Stucki depends more upon lubrication of side bearers than does Shannon.)

As a distinguished engineer has said, the great majority of derailments can be traced back to imperfections in the track surface, and that variations in cross level do produce lateral oscillations of the vehicles which, if not absorbed in the springs, must result in derailment.

The best protection against derailments is good track. Modern progress demands greater load's and higher speeds, and I feel that if engineers could only have the time to carefully investigate the true cause of accidents and remove them, the remarkable advance of recent years in speed and load could be even further improved upon. Beaton in his paper on Speed certainly had no doubt about it.

Early Morning at the Locomotive Running Shed, Auckland. (Photo, W. W. Stewart.)

Early Morning at the Locomotive Running Shed, Auckland. (Photo, W. W. Stewart.)

I trust that there will be something that will be useful in the above to our young railway men, and especially to those whose duty it may be to serve on Enquiry Boards.

If we have anything that will help our brother worker, let us pass it on, even if it is only a smile. If we can only induce our “modest wise ones” to come into print, there is a huge mass of workers only too thankful to receive the crumbs that fall from the rich man's table.

Rail or Lorry Cariage?

Whether the company's produce should go by rail or lorry to Moturoa was a question asked by Mr. Dempsey at a recent meeting of Kaponga suppliers.

The chairman said that the contract specified by rail from Eltham, but they had varied the contract and allowed five tons weekly to go direct by lorry. The price, however, was the same.

Mr. Dempsey considered that the lorry might be found to be the cheaper method.

The chairman referred to the extra wear on the roads and said that a large sum of the country's money was invested in railways, which they must maintain. From the railways they received a big concession on manure freights.

Mr. Perry: If one lorry load of produce goes over a bank it would cost us the whole of the year's saving in freights.

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