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Rail Wear on a Sharp Curve Explained
Travis S. Johnson
Maintenence of Way Superintendant
On a sharp curve, the lower (inside) rail is subjected to a fair amount of abuse and wear due to rather simple axel arrangements used in both rolling stock and locomotives. The top of the railhead takes the most abuse and on an older section of rail typically would be "spalled" or for lack of a more descriptive word, looking like it had tiny pieces of metal chipped out of the running band of the rail. This is due to the solid state of wheels and railroad axel. Unlike automobiles that use differentials to allow the right wheel to turn at a different speed then the left one, railroad wheels are connected by a rather substantial solid axel. Therefore the outside wheel must travel farther that the inside wheel. Since they are solidly connected, the inside wheel spins when rounding a curve.
With the enormous weight involved with locomotives and rolling stock, a surprising amount of cohesion exists and tiny flakes of steel are removed as this inside wheel "spins" against the low rail. Railheads, however, are not flat as many people might think think, but have a roundish profile that closely matches locomotives and rolling stock wheels. This profile changes in tangent track, the degree of the curve, and even the super elevation of the curve!
As measured by this 1860's track spirit level, the grade of the high (outside) rail of this curve is "super elevated" by 5" over the grade of the low (inside) rail.
As this rail flattens out from wear, the cohesion that exists that is "spalling" the railhead as the inside wheel spins becomes greater and greater, and thus more and more metal is removed. The only way to prevent such an occurrence is to make the inside wheel "spin" without causing damage. This is done through locomotives equipped with flange greasers, prematurely mounted rail greasers and mobile hi-railers that apply grease to the rails. This grease is also used on the "high rail," but for its own unique set of problems.
Here the laws of weight, gravity and kinetic energy have their own effect. Lets just say for instance a train is traveling a at fifty miles per hour and is entering a sharp right hand curve. Physics dictate that this fifteen thousand ton train wants to stay in a straight line. Only the placement of the rail, and the flanges on the rolling stock and locomotives wheels force this train to move in the direction of the track bed. Surprisingly, on perfectly ground rail (more on this later) and perfectly turned rolling stock and locomotive wheels, the amount of wheel flange to rail contact is 175 thousands of an inch, or about 3/16 of an inch. This contact only exists on the outside rail and tremendous pressure and heat is generated at the point of contact, hence the high pitched squealing often heard as a train tracks around a sharp curve.
Curve on Track #1 as it passes through Tunnel #3 (facing East) approaching MP 180 on the original Sierra grade near Cisco, CA.
Unfortunately, that is not the only forces acting
upon the rail. A tremendous amount of weight is also acting upon the top
of the railhead. This weight, coupled with massive kinetic energy of our
train traveling at fifty miles per hour, is literally forcing the steel
to move away from the inside edge of the rail. Granted this is taken place
at the molecular level, but over time, and with an influx of passing trains,
this can be visually seen. This is known as metal flow and can be seen
on both rolling stock and locomotive wheels, and on the railhead itself.
Because of the tremendous forces applied as our train enters the curve, the track structure (anchors, tie plates, spikes and cross ties) plays a very important part. Naturally with a redundancy of twenty five hundred times per mile, this system can have failures and still support a moving train, but wear is therefore accelerated. Worn ties and weak anchoring systems put more pressure on the railhead and over time can lead to premature railhead wear. Nowhere is this more apparent than at a mud hole. As mud hole erodes the ties that sit in it, the track compresses into the degraded ballast. In severe situations unknuckling can occur, but more than likely railhead wear will occur first. Because track structure is just that, a system, ties wear out prematurely as well from the rail and tie plates rubbing against them. This causes more compression, and more loose tie to rail connections, and thus even more compression.
As our train enters this mud hole, it slams the ties down into the mud hole crushing even more ballast from the hard hit, putting more pressure on the spikes and connections that hold the tie to the rail and "undercutting" the tops of the ties in sequence as well. If our train was a hundred cars long, this would occur four hundred times, or each time an axel rolled over that particular section of rail. Broken rails would be a common occurrence as well.
Wear to a modern steel railhead, Track #1 near Cisco, CA.
Mud holes and railhead wear
But just as mud holes abuse the track structure it does so to the railhead as well. Crushed heads can occur in defective rails, but corrugation is more likely to occur. Just as traveling down a muddy gravel road in an automobile with cause washboarding of the road surface, corrugation is the same with railroading. As the train enters the mud hole, the wheels are slammed against the railhead. Again on a molecular level, the steel of the railhead is compressed. Over time this causes corrugation which is little more than washboarding of the steel rail.
As each train passes this section of track, the corrugation becomes deeper and more pronounced, and telegraphs further and further down the length of rail. Now it is easily understood that such an occurrence is bad enough on tangent track where forces are generally equal on both rails, but on a curve, mud holes and corrugation place tremendous shock loads on the rail bed that are already subject to different and tremendous forces of their own.
Now it would seem the simple approach to dealing with mud holes since they are common and nearly impossible to cure, would be to create a track structure that is more impervious to wear. This is the direction the railroads took, but unfortunately the forefathers of railroading had the formula right all along. Concrete and steel ties were the obvious answer, but each has their own unique set of problems. Steel by its very nature, has tremendous tensile strength and its lack of compressive strength could be easily fortified by forming it in a bent shape. It was this bent shape however that became its downfall. Formed into a "U," steel ties held the gauge exceedingly well, but trapped mud. This mud came when the ballast that sat under the steel tie was broken by the weight of passing trains.
Unlike wood that erodes slowly and adsorbs this shock by compressing its wood fibers around this ballast, steel fragmented the rock. This fragmented rock thus trapped water and in a rather short amount of time created a mud hole where there was no mud hole before. Using standard tampers was also ineffective because they were not designed to "squeeze" the rock under these types of ties, and even if they could, that rock would soon be fragmented.
While grease is applied to rails by flange greasers to reduce wear on curves, locomotives also have to often drop sand under its wheels for traction while climbing steep grades.
Concrete would seem more idealistic, after all it is impervious to water, heavy and indestructible right? In a word, No!! Concrete has benefits and thus still used by Union Pacific and other railroads, but BNSF has discontinued their use in new construction. Their reasoning, accelerated railhead wear. Concrete is indeed heavy, and thus hold s the track down better than wood. Watching a train roll over wood ties, a wave can been seen as each Axel compresses the track bed into the ballast. With concrete this wave is far less pronounced, but is also the reason for its discontinuance on the BASF. Because this type of tie has no shock absorption, the railhead is literally pounded flat by the passing of trains and thus a whole new set of problems with the track structure begins to transpire. They are not as impervious to water as first thought however.
In the arid lands of western United States they work reasonably well in combating mud holes, but in the east, setting a concrete tie into a mud hole only exacerbates the problem. As the limestone inside the concrete tie leaches out, it fouls the ballast and thus makes the mud hole that much more pronounced. It also destroys the tie itself, and what might look like a perfectly good tie, is little more than a few inches thick in as little as five years!! Other limitations of concrete ties include their lack of being able to be placed by hand, destruction after a derailment and their high installation costs. As I stated earlier, our forefather's had it right. Wood ties are literally the shock absorbers of the track structure!
So how do railroads protect themselves from railhead wear? Surprisingly, by taking more off them. This is done by railgrinders. Mammoth, specially built machines that grind the steel using huge grinding wheels. They are programmed to grind the rail back to various profiles and have the ability to change where the running band lies in relationship to the super elevation of the track, degree of curvature, amount of wear, and type of wear. At a million dollars a mile for replacing the steel rails, spending twenty five thousand dollars a day to grind twenty miles or more of rail back to an acceptable level is very cost effective. It should also be noted, however, that new rail is also ground to take out imperfections and better match the running band to the region in which the rail is laid (curvature, super elevation, etc). Grease is of such importance to the railroads though, that they too are mobile rail greasers, pumping out gallons of track grease for every mile of rail they grind.
Travis Johnson, Belfast, Maine, July 6, 2005.
S. Johnson. [11/27/2004]
Original digital photography and illustrations by Bruce C. Cooper.
Examples of severe rail wear on sharply curved track.