tdmidgetBucky, you are hopeless. The lag screws DO NOT BREAK ABOVE THE THREADS. They break IN THE MINOR DIAMETER. That is the weakest point of any threaded fastener and is worse here because of the coarse pitch of the threads. The lag screw is a bad idea.
Yes they do break in the minor diameter which is the weakest point. My point was that they do not strip out of the wood, so a lack of grip is not the problem. I have edited my above post to address your concern about my point which I characterized as "above the threads."
Bucky, you are hopeless. The lag screws DO NOT BREAK ABOVE THE THREADS. They break IN THE MINOR DIAMETER. That is the weakest point of any threaded fastener and is worse here because of the coarse pitch of the threads. The lag screw is a bad idea.
The SSRS might work. It does not have a minor diameter as a weak point but it still offers no real resistance to lateral movement. The greater mass of metal will likely be stronger but only testing will prove that.
If I was UP I would get them in place somewhere like powder river area with lots of heavy trains and no or little hazmat and find out how godd they are. Lag screws would be out, cut spikes in.
Overmod had posted this link earlier to demonstrate an interesting new approach to rail anchors. It is called the SELF-SECURING RAIL SPIKE.
Interestingly, the benefit claimed for this fastener is better holding power. It features a rather ingenious anchoring principle by which the legs of a two-legged pin are wedged to spread apart when driven into the tie. The legs have bevels on their ends, and the wedging action of the bevel passing through the wood forces the legs to spread. In spreading, the two legs act as a loading spring. So when fully spread, the legs maintain an active grip on the wood between them. Additionally, the legs have small barbs on the inside of the legs.
The idea seems that it would be very effective.
But is added holding power really what is needed? I don’t think so.
The problem with lag screws is that they don’t have the column strength; that is the ability to withstand the shear loading perpendicular to their axes without bending.
So they bend and the bending also induces vertical pull. They bend and pull until they fatigue and break. There is no shortage of holding power. This is proven by the fact that they always break above most of the screw threads. The screw threads are doing their job just fine. That part of the screw stays anchored in the tie when the screw breaks.
Why won’t these Self-Securing Rail Spikes break off just like the lag bolts? They have no enhanced features that would make them less prone to breaking off above the grip of the two spring loaded legs.
The SSRSs do have a slight advantage over lag bolts in that they compress the wood rather than subject it to tensile stress as a lag screw would do in tending to split the tie from the expansive force of the interfering threads. But, again, lag screws do not lack sufficient holding power, even if they do tend to cause splitting.
As I mentioned earlier, I believe that lifting is a consequence of bending. If bending occurs, it will lift these Self-Securing Rail Spikes despite their barbs gripping the wood. It will either tear the wood to release the barbs, or it will bend and pull until the upper portion of the device breaks off. So it will do no good to add features that discourage lifting while doing nothing to discourage bending. The problem will be solved only by increasing the resistance to bending.
The Self-Securing Rail Spike:
http://www.pages.drexel.edu/~garfinkm/Spike.html
Paul of CovingtonInstead of the elaborate scheme of lag srews and bushings where you would have to drill for the screw and counter-bore for the bushing, why not just go with considerably larger lag screws. The larger diameter would have an effect similar to the bushing, not to mention the much increased strength in tension and the greater area for the threads to hold in the wood.
Paul,
A larger bolt might be the solution. I think it depends on how much larger the diameter needs to be in order to provide a sufficient lifespan. The overall problem seems to be a lifespan that is too short, and not a matter of unpredictable, random failure along the way. When the life runs out, there is mass failure.
At this time, the screw diameter is 15/16”. If sufficient life could be achieved by increasing the diameter to say 1-1/8”, maybe making the screw that diameter would be the best choice.
But if the shank diameter needs to be 1-1/2” to provide sufficient life, then a bushing of that O.D. would be the better choice, because an entire lag screw of that size would provide threads that would probably be overkill in holding power, and their cutting the corresponding thread into the tie might split it.
I believe that the present screw size is sufficient in terms of thread holding ability, but the upper unthreaded shank needs a larger diameter. This would stiffen the shank directly to reduce bending and provide more bearing area in the tie to further reinforce the shank against bending.
There are many different combinations of features that might solve the problem. These need to explored and tested. I don’t think that task has been completed. People who did not like the detailed engineering minutia probably decided that pursuing the matter to its intelligent conclusion amounted to beating a dead horse; so they just cut off and went with a half-baked idea.
Maybe add a lock-washer to maintain tension as the tie shrinks.
Instead of the elaborate scheme of lag srews and bushings where you would have to drill for the screw and counter-bore for the bushing, why not just go with considerably larger lag screws. The larger diameter would have an effect similar to the bushing, not to mention the much increased strength in tension and the greater area for the threads to hold in the wood.
_____________
"A stranger's just a friend you ain't met yet." --- Dave Gardner
Overmod
I conclude that maintaining sufficient clamping pressure is not the key to preventing lateral movement of the tie plate. I don’t believe that it would be possible to maintain sufficient clamping pressure with lag screws threading into a timber tie. As you mentioned a while back, the slightest relaxation of the clamp effect loses all of the pressure.
So, I think the key to preventing lateral movement of the tie plate is to have the bolts functioning only as solid pins perpendicular to the tie plate resisting the shear force. Properly designed, that function will persist reliably over time.
Right now, there is a combination of clamping pressure to create shear friction, and lag screws to act as shear pins; but neither one, nor both combined, are adequate to do the job. So I would get rid of relying on the clamping pressure function, and strengthen the bolt shear resistance by adding the big bushing.
The big bushing would have a pilot to fit the hole in the tie plate, and a bore to fit the bolt shank. These fits would not need to be especially tight. They could be loose enough to accommodate the manufacturing tolerance variation of the hole patterns of the plate and tie.
But the point of the bushing is to increase the effective diameter of the screw shank so it has a much larger load bearing surface area on the bore surface of the tie hole. That larger load bearing surface would overcome the destructive potential of the limited plate shifting allowed by the clearance of the fits of bolt-to-bushing and bushing-to-plate.
Norm48327 There are simple solutions for complex problems and most of them are wrong.
There are simple solutions for complex problems and most of them are wrong.
Off topic, but the simplest solution seems to solve the complexest problem:
Norm
Overmod,
Earlier you questioned my mention of the uplifting force on the screw causing its threads to strip out of the tie. You are right in your conclusion. In thinking about it, this uplifting force is not stripping the threads out of the wood. This is evidenced by the fact that the break leaves most of the threaded portion intact in the tie. The wallowing out of the hole by the cyclic bending probably compromises the first few threads at the top, but is probably not instrumental in actually stripping the lower portion of the bolt’s proper fitting threads out of the wood. However, the lifting force pulses on the lag bolt may be capable of unscrewing the screw from the wood.
My uncle Guido takes care of them for me.
Euclid Norm, I am amazed that I can inflict such pain on you just by getting into a little detail here. How do you cope with real problems?
Norm,
I am amazed that I can inflict such pain on you just by getting into a little detail here. How do you cope with real problems?
EuclidYes I do realize that the screw hole in the plate is not reamed for a tight fit to the screw shank. It would be nice if it was, but the plate holes and tie holes are in a pattern and the patterns must match. It would be interesting to know what the fit of the plate hole to the screw shank is. I would guess the hole is at least about .040 over in the diameter. But the lateral movement of the plate that shows up in the photos must be at least a half inch. I would not be surprised if it can move that far without breaking any screws. I agree that the visual appearance of the plate movement gives no indication of what is happening below, but I would be very curious, and would open one up to find out. As to your question about whether lifting has been observed, I don’t know if it has. I only conclude that it exists and contributes the bending fatigue that ultimately causes the breaking. Here is why I conclude that lifting exists: When you bend the screw with the corresponding shift of the tie plate, the elevation of its head must drop vertically. But it cannot drop because it is being upheld by the tie plate. If the lower part of the screw threads is still holding, the screw cannot rise from the tie to accommodate the reduction in vertical distance due to bending of the shank. So the reaction to bending the screw causes a lifting force against the bottom of the head that is resisted by the threads still fully engaged in the lower part of the screw. It is the bending force that lifts the screw. I am not convinced that even the maximum clamping pressure on a new installation is sufficient to prevent the onset of tie plate lateral shifting. But even if it is, I think that sufficient clamping force is quickly lost just by the plate compressing the wood due to the vertical compression. So, overall, I see these forces and their affects culminating in the final breaking of the screw: Lateral force of the tie plate upsets and abrades the screw shank under the head, and around the hole in the tie plate. Lateral force of the tie plate bends the screw shank in a range extending downward from the head. Bending of the screw shank causes crushing and ovaling of the tie hole, and damages thread engagement. Bending of the screw shank causes fatigue cracking in the screw shank near the weak point where the top of the threads begin. Bending of the screw shank causes upward lift of the screw which contributes to the fatigue cracking induced by bending.
Sheesh Bucky. I bet it takes you two hours to eat breakfast because you analyze each and every flake of oatmeal in your bowl.
Time to give us a break.
schlimmNah! At the risk of sounding sexist, we need the moment when "the fat lady sings" i.e., the end is nigh!
I know one who is not fat, just pleasingly plump, but has a great soprano voice. When can I bring her on stage?
Technically at least it would be possible to gang-drill the tie directly through the plate so that a reamed bolt would 'line up' reasonably well (or would reasonably self-align as the head came down to clamping range). The problem is that you'd have zero gauge adjustment other than shimming the clipped seats thereafter, and any deformation of the tie seat relative to the plate would throw additional bending stress down into precisely the area of the bolts where we're now apparently seeing the accelerated failures.
A potential concern with 'hard' bushings in the ties is that they'd have to align very precisely with the plate holes to be any good. I would note by analogy that this is similar to the concern between lug-centric and hub-centric wheel fits on motor vehicles, and I'd repeat that some mechanical means of restraining the lateral (probably with a pin-and-bushed-hole arrangement completely independent of the screw fixation) is going to be the 'best' overall kind of answer -- might even be done with cut or hairpin spikes...
Once you have eliminated the plate laterally striking the shank of the screw, or tending to deflect it, you have solved most of the 'bending' or 'bowing' issues for the screws, and can concentrate much more on maintaining effective clamping force, without having to machine careful fits or overconstrain part of the track geometry.
Something else to consider about your proposed mechanism is that I have yet to see a reported 'catastrophic' failure of these lags in a gauge accident that involves the heads popping off -- and if I recall, the Fabyan Bridge report makes specific mention of this and why it is curious. Any sort of rotation of the plate (as for example that which presumably occurs as the seat in the tie begins to 'cut' or crush downward) will put tremendous spot pressure on the edge of the head immediately adjacent to the almost-square 'corner' between the clamping face of the head and the adjacent shank. Any microcracking here will tend to repetitively open, fill with water or debris, and start 'jacking itself open' as well as providing a very likely environment for SCC. Preferential head failures are noted in other environments, so there is no 'magic' for the lag fixation here. And you would see this, I think, long before you observed 'macro' pulling of all those threads out of the tie wood (which has yet to be established, and I think in fact we won't see it much).
Of course rotating the plate relative to the head is going to reduce the clamping pressure on a considerable portion of the expected clamping area, which will likely facilitate some sliding and selective wear of the underside of the heads. If we do not see this, it's an indication that the resultant of stretch and bending is being communicated to the 'critical' breaking zone, a couple of threads below the initial transition area, and augmentation there might be the most 'logical' place to start with a redesign analysis.
Overmod Euclid I surmise that before a screw breaks, it does a lot of bending cycles. It might bend several million times before it finally breaks from fatigue. In order to bend, the screw shank has to crush the wood on the side of the screw hole opposing the bend movement. So I conclude that the presence of exposed polished tie surface indicating the tie plate moving back and forth does not necessarily mean that any or all of the screws have broken. Look a little more carefully at the physics. The hole in the plate is not reamed precisely to a fit with the shank. At the same time, the bending and plate shifting exert an upward pull on the screw which begins stripping the threads out of the wood. Has this actually been observed in the bolts that are breaking?
Euclid I surmise that before a screw breaks, it does a lot of bending cycles. It might bend several million times before it finally breaks from fatigue.
In order to bend, the screw shank has to crush the wood on the side of the screw hole opposing the bend movement. So I conclude that the presence of exposed polished tie surface indicating the tie plate moving back and forth does not necessarily mean that any or all of the screws have broken.
Look a little more carefully at the physics. The hole in the plate is not reamed precisely to a fit with the shank.
At the same time, the bending and plate shifting exert an upward pull on the screw which begins stripping the threads out of the wood.
Has this actually been observed in the bolts that are breaking?
Yes I do realize that the screw hole in the plate is not reamed for a tight fit to the screw shank. It would be nice if it was, but the plate holes and tie holes are in a pattern and the patterns must match. It would be interesting to know what the fit of the plate hole to the screw shank is. I would guess the hole is at least about .040 over in the diameter. But the lateral movement of the plate that shows up in the photos must be at least a half inch. I would not be surprised if it can move that far without breaking any screws. I agree that the visual appearance of the plate movement gives no indication of what is happening below, but I would be very curious, and would open one up to find out.
As to your question about whether lifting has been observed, I don’t know if it has. I only conclude that it exists and contributes the bending fatigue that ultimately causes the breaking. Here is why I conclude that lifting exists:
When you bend the screw with the corresponding shift of the tie plate, the elevation of its head must drop vertically. But it cannot drop because it is being upheld by the tie plate. If the lower part of the screw threads is still holding, the screw cannot rise from the tie to accommodate the reduction in vertical distance due to bending of the shank. So the reaction to bending the screw causes a lifting force against the bottom of the head that is resisted by the threads still fully engaged in the lower part of the screw. It is the bending force that lifts the screw. I am not convinced that even the maximum clamping pressure on a new installation is sufficient to prevent the onset of tie plate lateral shifting. But even if it is, I think that sufficient clamping force is quickly lost just by the plate compressing the wood due to the vertical compression.
So, overall, I see these forces and their affects culminating in the final breaking of the screw:
Lateral force of the tie plate upsets and abrades the screw shank under the head, and around the hole in the tie plate.
Lateral force of the tie plate bends the screw shank in a range extending downward from the head.
Bending of the screw shank causes crushing and ovaling of the tie hole, and damages thread engagement.
Bending of the screw shank causes fatigue cracking in the screw shank near the weak point where the top of the threads begin.
Bending of the screw shank causes upward lift of the screw which contributes to the fatigue cracking induced by bending.
We already have this -- but the lady involved is far from fat!
Overmod schlimm In the spirit of these Scholasticist lines one could ask whether it is helping "his thought process constructively" or merely enabling the dysfunctionality? I do ask myself that. Sometimes repeatedly. But perhaps one of the dead horses may learn to sing.
schlimm In the spirit of these Scholasticist lines one could ask whether it is helping "his thought process constructively" or merely enabling the dysfunctionality?
I do ask myself that. Sometimes repeatedly.
But perhaps one of the dead horses may learn to sing.
Nah! At the risk of sounding sexist, we need the moment when "the fat lady sings" i.e., the end is nigh!
C&NW, CA&E, MILW, CGW and IC fan
Overmod Euclid What number would you use so as not to distract from attention from my valid point? I'm not certain every wheel passing represents a full lateral load excursion; I'm waiting for some more defined physics and geometry testing to get some idea of how, and when, the lateral excursions in the loosened fixation system behave. I was concerned that some readers would see 'several million' and start to overreact. Perhaps 'up to several million' captures the possibilities without invoking too much undue "wrath."
Euclid What number would you use so as not to distract from attention from my valid point?
I'm not certain every wheel passing represents a full lateral load excursion; I'm waiting for some more defined physics and geometry testing to get some idea of how, and when, the lateral excursions in the loosened fixation system behave. I was concerned that some readers would see 'several million' and start to overreact. Perhaps 'up to several million' captures the possibilities without invoking too much undue "wrath."
Well, I did not get into the fine point of how to count the wheels in the affect, so I counted all four wheels in the two adjacent trucks as one rail-pushing cycle. But even that gets into several million cycles. Believe me, I gave deep thought to the use of the number because I just knew that several readers would take it as an overreaction or exaggeration. So I was ready.
Overmod I'm not certain every wheel passing represents a full lateral load excursion; I'm waiting for some more defined physics and geometry testing to get some idea of how, and when, the lateral excursions in the loosened fixation system behave.
I'm not certain every wheel passing represents a full lateral load excursion; I'm waiting for some more defined physics and geometry testing to get some idea of how, and when, the lateral excursions in the loosened fixation system behave.
generally only the lead axle of a truck would exercise a fastener unless the truck is misbehaving.
There are curving detectors out there to identify "bad actors"
EuclidWhat number would you use so as not to distract from attention from my valid point?
You spell laterally the way I corrected it, not the way you had it in your original prose.
Overmod Euclid I surmise that before a screw breaks, it does a lot of bending cycles. It might bend several million times before it finally breaks from fatigue. You might want to backcheck and see how many trains it would take to deflect a screw severely 'several million times'. The number deflects attention from your valid point,...
You might want to backcheck and see how many trains it would take to deflect a screw severely 'several million times'. The number deflects attention from your valid point,...
Depending on how you factor the number of wheels into the number of cycles: Ten trains per day at 100 cars each = 1000 cycles. That is 365,000 cycles per year. In ten years, that is 3,650,000 cycles. That is what I mean by “several million.”
If you count each wheel as a load cycle, it is 14,600,000 cycles in ten years. So you might say it is 3-14 million. I thought “several million” was a fair way to capture the general idea.
What number would you use so as not to distract from attention from my valid point?
And how do you spell laterally?
schlimmIn the spirit of these Scholasticist lines one could ask whether it is helping "his thought process constructively" or merely enabling the dysfunctionality?
Thank you.
Overmod Norm48327 Have you not already taken this subject far beyond what any reasonable person would? The beating of the dead horse continues. Let go of it. Let the poor fellow think out loud if he wants to -- and help his thought process constructively, if you can. No one is twisting your arm to read the horse-beating.
Norm48327 Have you not already taken this subject far beyond what any reasonable person would? The beating of the dead horse continues. Let go of it.
Let the poor fellow think out loud if he wants to -- and help his thought process constructively, if you can.
No one is twisting your arm to read the horse-beating.
In the spirit of these Scholasticist lines one could ask whether it is helping "his thought process constructively" or merely enabling the dysfunctionality?
Norm48327Have you not already taken this subject far beyond what any reasonable person would? The beating of the dead horse continues. Let go of it.
Euclid I surmise that before a screw breaks, it does a lot of bending cycles. It might bend several million times before it finally breaks from fatigue. In order to bend, the screw shank has to crush the wood on the side of the screw hole opposing the bend movement. So I conclude that the presence of exposed polished tie surface indicating the tie plate moving back and forth does not necessarily mean that any or all of the screws have broken. All it indicates is that the tie plate frequently moves in and out laterally in relation to the tie. As screw holes enlarge from the bending force crushing the wood, and the loss of anchoring of the screw, the tie plate is free to shift latterly. So the tie hole is progressively ovaled due to the bending of the screw. Each bend is progressively larger, and makes the oval slightly deeper. At the same time, the bending and plate shifting exert an upward pull on the screw which begins stripping the threads out of the wood. This loss of threads then combines with the ovaling of the hole; and further reduces the resistance to tie plate shifting. Thus the tie plate shifts further with each bend cycle, and becomes capable of considerable lateral movement without breaking the screw. Finally, as the oval widens; the screw bends further; the number of completed bend cycles grows larger; fatique cracking develops, and the screw finally breaks. This evidence of tie plate shifting would be easy to spot while walking, and it seems to me that the degree of shifting evident in the published photo would be more than adequate to raise the alarm of a serious securement problem. In observing the evidence of tie plate lateral movement, one would have to conclude that the gage is not being properly constrained. I doubt that one would find such plate movement evidence as shown in the photo with just one isolated tie. Several adjoining ties would be most likely to exhibit the same condition. The tie plate moves because the rail moves. So how could the rail move that far and carry just one tie plate? In the Fabyan Bridge wreck, investigators found plenty of evidence indicating an imminent failure by just looking closely at the track. Such evidence included broken tie plates and broken screws that had been dislodged from their holes and were lying there in plain sight. Apparently, however, assuming track inspectors had noticed these problems, they did not perceive it as evidence of imminent failure. Instead, they must have regarded the problem as random and limited individual breakages.
I surmise that before a screw breaks, it does a lot of bending cycles. It might bend several million times before it finally breaks from fatigue. In order to bend, the screw shank has to crush the wood on the side of the screw hole opposing the bend movement. So I conclude that the presence of exposed polished tie surface indicating the tie plate moving back and forth does not necessarily mean that any or all of the screws have broken. All it indicates is that the tie plate frequently moves in and out laterally in relation to the tie. As screw holes enlarge from the bending force crushing the wood, and the loss of anchoring of the screw, the tie plate is free to shift latterly.
So the tie hole is progressively ovaled due to the bending of the screw. Each bend is progressively larger, and makes the oval slightly deeper. At the same time, the bending and plate shifting exert an upward pull on the screw which begins stripping the threads out of the wood. This loss of threads then combines with the ovaling of the hole; and further reduces the resistance to tie plate shifting. Thus the tie plate shifts further with each bend cycle, and becomes capable of considerable lateral movement without breaking the screw. Finally, as the oval widens; the screw bends further; the number of completed bend cycles grows larger; fatique cracking develops, and the screw finally breaks.
This evidence of tie plate shifting would be easy to spot while walking, and it seems to me that the degree of shifting evident in the published photo would be more than adequate to raise the alarm of a serious securement problem. In observing the evidence of tie plate lateral movement, one would have to conclude that the gage is not being properly constrained.
I doubt that one would find such plate movement evidence as shown in the photo with just one isolated tie. Several adjoining ties would be most likely to exhibit the same condition. The tie plate moves because the rail moves. So how could the rail move that far and carry just one tie plate?
In the Fabyan Bridge wreck, investigators found plenty of evidence indicating an imminent failure by just looking closely at the track. Such evidence included broken tie plates and broken screws that had been dislodged from their holes and were lying there in plain sight.
Apparently, however, assuming track inspectors had noticed these problems, they did not perceive it as evidence of imminent failure. Instead, they must have regarded the problem as random and limited individual breakages.
Have you not already taken this subject far beyond what any reasonable person would? The beating of the dead horse continues. Let go of it.
EuclidI surmise that before a screw breaks, it does a lot of bending cycles. It might bend several million times before it finally breaks from fatigue.
You might want to backcheck and see how many trains it would take to deflect a screw severely 'several million times'. The number deflects attention from your valid point, that there is a 'common mode' failure in these screws for which an explanatory mechanism is needed stat.
Look a little more carefully at the physics. The hole in the plate is not reamed precisely to a fit with the shank. So as soon as the effective clamping force is limited, the plate can slide relative to the shank, and does. Even slight motion with the associated lateral force will produce high acceleration and, probably, high contact force between the edge of the hole and the screw shank. This may be augmented by effects of the spring clip that holds the rail in the seat on the plate. I'm looking forward to seeing some instrumented data on this.
[quote ... As screw holes enlarge from the bending force crushing the wood, and the loss of anchoring of the screw, the tie plate is free to shift laterally (note sp.) ... the tie hole is progressively ovaled due to the bending of the screw. Each bend is progressively larger, and makes the oval slightly deeper.[/quote]
I suspect lateral motion may not be the only kind experienced by the bolts. Rail anchors may not preclude longitudinal motion to an extent that causes displacement of the shank, perhaps in a 'resultant' direction. But this, too, awaits more specific analysis.
Has this actually been observed in the bolts that are breaking? I had thought the problem was that the threading was effectively overconstraining the bottom of the bolt, keeping any ovaling or lifting at all from reaching most of the threaded portion (unless the tie were suffering 'killing' from water and foreign-matter incursion along the enlarged portion of the bore). That is a far more serious effect than if the whole bolt were 'working'.
... the tie plate shifts further with each bend cycle, and becomes capable of considerable lateral movement without breaking the screw. Finally, as the oval widens; the screw bends further; the number of completed bend cycles grows larger; fatique cracking develops, and the screw finally breaks.
I asm not convinced this is as extended or cumulative a process as that. Most of the lateral bending effects will commence with some severity as soon as the effective clamping preload is lost. What I'd start looking at instead would be some of the mechanisms by which relaxation of lateral resistance in some bolts puts additional load on intact screw fixation clamping, in a direction the loading system is not designed to resist.
This evidence of tie plate shifting would be easy to spot while walking, and it seems to me that the degree of shifting evident in the published photo would be more than adequate to raise the alarm of a serious securement problem.
I think this is one of those situations where 'it's easy to spot now that we know what to look for'. Where the problem appears to be arising is what to do when the problem is observed -- it appears that just periodic redriving of the bolts, as one would do with cut spikes, is not an acceptable answer whether or not you remediate broken bolts when you come across them during the redriving process.
Part of the problem appears to me to be that the 'cause' of adjacent plates moving may not be 'the same' as the movement from an initial lateral clamping failure, so there may not be the distinctive signature for a trackwalker to note. It is true that you'd need several adjacent plates loose to cause a 'critical' widening of gauge; on the other hand, once the gauge widens enough in any spot to start dropping a wheel, catastrophic progression of lateral shift and gauge widening, even if the rail doesn't roll in the clips, is likely to follow.
In the Fabyan Bridge wreck, investigators found plenty of evidence indicating an imminent failure by just looking closely at the track. Such evidence included broken tie plates and broken screws that had been dislodged from their holes and were lying there in plain sight. Apparently, however, assuming track inspectors had noticed these problems, they did not perceive it as evidence of imminent failure. Instead, they must have regarded the problem as random and limited individual breakages.
You seem to have forgotten that in the TSB report it is clearly and repeatedly stated that the track was 'borderline' critical, but hadn't "quite" reached the trigger point using standards appropriate to spiked track. Track inspectors probably quite rightly 'regarded the problem' as a widespread inability to hold gauge, with evidence of bad ties, loose hardware, and other associated problems. The real issue I saw was that the result of such failures was, and is, far more critical with screw fixation than with spikes, in part I think because of a tacit assumption that using four big screws per plate instead of two scrawnier hammered spikes just had to mean far more securement.
I suspect there is relatively little 'shared lesson' between the maintenance procedures used at Fabyan Bridge and at Mosier. That may change if the NTSB investigation shows any kind of not-quite-critical maintenance alert for the whole stretch of track, but from what I've been given to understand UP was reasonably diligent in performing both routine and preventative maintenance on a 'reasonable' basis. The emergent issue is that, for this track-fixation system, the reasonable basis does not seem to be adequate.
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