What would cause freight cars to separate in the curve and derail?

Question

What would cause freight cars to separate in the curve and derail?

Answer 1

one of the major causes(and Hoghead alluded to this) is a condition known as "rigid trucks"  trucks that because of improper maintenance are not functioning correctly and will not properly track through a curve.  The other that Hoghead mentioned is the wheel tread profile as it interfaces with the rail head.  An improper wheel tread profile will lead to what is called truck hunting which is very bad at speed.  It is caused by (again, as Hoghead mentioned) the difference in effective wheel diameters across the surface of the tread.

Answer 2

This happens because of LOV.  

No.  Not that kind of ‘love’.

Curvature is a tricky business where most modes of transportation are concerned, but it is particularly problematic for railroad operations.  That is why one of the most common places for a derailment to occur is on a curve.

The problems don’t arise strictly from the viewpoint of ‘tracking’ through a curve.  The axles don’t have differentials as do automobile, that allows the outside wheel to turn faster than the inside wheel when traversing a curve.  In the absence of that differential, the wheels themselves are ‘tapered’ to allow for some compensation, which helps the equipment travel through the curve with less binding and, in effect, allowing the outside wheels to travel further when compared to the wheels on the inside.  But not all of the ‘flange bind’ is eliminated, its effects are only mitigated.

The true difficulty presents itself in the form of “track / train dynamics.”  This is the name for the forces that are present where wheel meets rail, which is where the forces are the highest.  Ã¢Â€ÂœLOV,” is the acronym for the ratio between the vertical forces and the horizontal forces that act on equipment as they traverse any piece of track, but is more acute in curvature.

Keep in mind that the flanges’ on the inside of the wheels, main purpose is to force the wheels to follow the rails.  The force that keeps the equipment on top of the rail is derived only from how much the piece of equipment weighs, which is the downward force, or, “V”ertical force.  The forces that acts on the wheels through the curvature is sideways, or horizontal, “L”ongitudinal force.

The ratio is expressed as L / V, or “L” over “V”.  As long as the vertical “V” force is greater than the longitudinal, or “L” force, the equipment will stay on top of the rail and allow the flanges to guide the equipment through the curve.  Once we know this, we can understand the track / train dynamics.

The problems arise, and derailments occur, when the “L”ongitudinal forces exceed the “V”ertical forces, which will cause the wheels to want to climb the outside of the rail, or when longitudinal forces are excessive and pull equipment off of the inside of the curve.  Now all we need is to know what can cause these excessive forces.

Slack runs through trains.  There is play between couplers of the cars, and many are equipped with cushioning devices that allow for greater slack between the cars.  As slack changes and runs through the draft gear, it gains momentum as it travels, much like cracking a whip.  This slack action can cause ‘spikes’ in the force levels.  If excessive, and in curvature, the L/V ratio is drastically imbalanced.

You can prove the principals to yourself

Take any kind of chain, bicycle, links, string even.  Lay it out on a tabletop, and push on each end.  It crumples up, doesn’t it?  The same thing happens when there is a spike in the compressive forces in the train, when the slack is “bunched,” usually when dynamic brake is used.  If the compressive forces (buff) are too great, the lateral forces spike, and the equipment will leave the track, toward the outside of a curve.  This is called “jack-knifing.”

Now, take a piece of string and lay it out on your table top, with some curves in it.  Get something light to place on the inside of each curve that will still allow for the string to be pulled through, sugar cubes, thimble, chess pawns, anything.  Pull on one end of the string.  The rest of the string will follow around the pieces you have set up.

Now, put a finger on one end of the string keeping it from moving, and pull on the other.  It won’t move around the pieces and more.  It goes to a straight line, and knocks over whatever you have set up.  This phenomenon is called, appropriately enough, “string-lining.”  In this instance, where the slack is stretched, with an engine pulling on it (draft), if the forces are too great, the equipment will be pulled off of the track on the inside of the curve.

These situations are particularly bad in grade territory, where compressive forces in trains running downhill, and draft forces are very high heading up hill, that curvature contributing to adverse track / train dynamics is highest.

In your question, you mentioned a separation of the train in curvature and resultant derailment.  A separation can occur first, causing the train air-brakes to make an ‘emergency” application.  This alone will cause the force levels to skyrocket, and the result can be a derailment.

Or, the force levels can be present to cause the derailment, which of course results in separation.  That is why, when trying to determine the cause of a derailment, it is so important to know which happened first, since one can cause the other.

Excessive speed can create the same excessive force on the outside rail of a curve, and the flanges of the wheels are going to climb the rail and there will be a derailment.  But, cases of excessive speed are rare, in and of themselves, unless from a loss of braking capacity for any reason.

I have gone into this in detail in a five part article that I have posted on my blog, and how these principals contributed to the disastrous derailment at Cantara Loop in 1991, that killed off a 25+ mile stretch of the Sacramento River in Northern California.  You may find it interesting.  Just click an HOGHEAD and go to my 360, and start with the post of February 25, 07.  It is explained in much more detail and more easily understandable.

A really good question, by the way...............

Answer 3

Broken coupler, sink hole

Answer 4

Maybe going too fast? Rail became loose? Wheel came off or locked up? Something on the track?  Faulty brake system or coupler?

Answer 5

warped tracks.

Answer 6

Don't they separate after they derail?