Heat Restrictions in March?

[Horizon22]

I am aware that several trains from Plymouth have been delayed over the last few days due to speed restrictions between Westbury and Newbury. For example this train losing 10 minutes in the area. However, why would their be speed restrictions at this time? The temperature has been unseasonably warm yes, but only just reaching 20 degrees. Summer temperatures are much higher and even then only on particularly warm days in do we normally see speed restrictions across the network when the rail temp can't cope above its normal threshold.

So why would these be imposed at temperatures at a maximum of 18 degrees C? Or would it be that the track is generally not in the best condition? It's concerning that if its happening under these circumstances, that in a few months time we could see a whole plethora of restrictions which will severely reduce performance.

 

[Snow1964]

Was the track recent relaid or rerailed

There have been occasions where works have run out of time and the stressing of rails wasn’t completed, not a problem in the winter, but they have to come back and finish it before warmer weather.

I wonder if this is still awaiting full stressing, but as the warmer weather was forecast, it’s unclear why it wasn’t done.

 

[Bald Rick]

You can get them in February - I had to do that once and caused a very lively conversation with some operating people. Basically it’s the laws of physics. I think I wrote something about the subject a few years ago.

The very short version is that it is temperature range that is the problem, not the top temperature. At this time of year, rails can be installed at sub zero temperatures, and I bet rail temperature was 30C somewhere yesterday.

The short version is that if trackwork is done that disturbs the underlying track, it is less ‘fixed’ in place. The expansion of rails in the heat is usually prevented on continuously welded rail by stressing the rail (physically stretching it with big jacks) which means that when warmed up the rail just gently unstresses itself. However for some jobs the stressing can’t be done at the same time as the rest of the work. Big expansion in a wide temperature range increases the risk of rail buckles, and therefore speed restrictions are put in place to a) reduce the stresses when traffic passes, and b) minimise the consequences of a derailment if one does happen. Worst case is when there is new track on new ballast, on a curved site, with no stressing.

 

[JN114]

Rail Temperature can be considerably higher than ambient air temperature, especially in direct sunlight as we’ve had over the past couple of clear/fine days.

 

[AM9]

Rail Temperature can be considerably higher than ambient air temperature, especially in direct sunlight as we’ve had over the past couple of clear/fine days.

I remember posts in previous threads here quoting 27/28 deg C as the neutral stress temperature. With the effects of climate change is the dafault temperature being eased upwards in certain locations?

 

[Bald Rick]

I remember posts in previous threads here quoting 27/28 deg C as the neutral stress temperature. With the effects of climate change is the dafault temperature being eased upwards in certain locations?

It’s still 27C; an upward lifting has been considered, but the data doesn’t support it. Interestingly in France it is 28C. When you lift the stress free temp (SFT), even by 1C, it increases the risk of broken rails on cold days. Also, and not insignificantly, it would take a decade or more to get the SFT up nationwide, unless there was a wide scale (and expensive, and disruptive) restressing exercise.

 

[AM9]

It’s still 27C; an upward lifting has been considered, but the data doesn’t support it. Interestingly in France it is 28C. When you lift the stress free temp (SFT), even by 1C, it increases the risk of broken rails on cold days. Also, and not insignificantly, it would take a decade or more to get the SFT up nationwide, unless there was a wide scale (and expensive, and disruptive) restressing exercise.

I understand the hot vs cold limits of track, - is there an option to select an alloy with a different CoE for the rail (presumably at a cost but that might be mitigated by the reduced necessity to adjust the tension twice yearly). On the subject of network-wide introduction, I would assume that any changes would be done progressively as track needed rail replacement, maybe concentrating on the worst hot spots (pun intended ) first.

 

[Bald Rick]

I understand the hot vs cold limits of track, - is there an option to select an alloy with a different CoE for the rail (presumably at a cost but that might be mitigated by the reduced necessity to adjust the tension twice yearly). On the subject of network-wide introduction, I would assume that any changes would be done progressively as track needed rail replacement, maybe concentrating on the worst hot spots (pun intended ) first.

I don’t think there’s anywhere in this country that changes the SFT twice yearly. That is done on other railways of course. Not sure about a different CoE.

Were it to be changed, then I’d expect it to be done as and when rail is replaced for whatever reason.

 

[Horizon22]

You can get them in February - I had to do that once and caused a very lively conversation with some operating people. Basically it’s the laws of physics. I think I wrote something about the subject a few years ago.

The very short version is that it is temperature range that is the problem, not the top temperature. At this time of year, rails can be installed at sub zero temperatures, and I bet rail temperature was 30C somewhere yesterday.

The short version is that if trackwork is done that disturbs the underlying track, it is less ‘fixed’ in place. The expansion of rails in the heat is usually prevented on continuously welded rail by stressing the rail (physically stretching it with big jacks) which means that when warmed up the rail just gently unstresses itself. However for some jobs the stressing can’t be done at the same time as the rest of the work. Big expansion in a wide temperature range increases the risk of rail buckles, and therefore speed restrictions are put in place to a) reduce the stresses when traffic passes, and b) minimise the consequences of a derailment if one does happen. Worst case is when there is new track on new ballast, on a curved site, with no stressing.

Thanks, that’s pretty comprehensive. Wasn’t aware it was the range and always believed it was the absolute temperature. As you say we’ve had some considerable range recently.

Presumably therefore once the track has “bedded in” over next few weeks, these restrictions should be removed.

 

[Bald Rick]

Presumably therefore once the track has “bedded in” over next few weeks, these restrictions should be removed.

Absolutely. The Critical Rail Temperature (which is a function of the Stress Free Temperature) rises a couple of degrees each week with the passage of traffic. And of course stressing (if it hadn’t been done) lifts it much further.

Also, it’s got colder

 

[mr_moo]

I agree with the others. It's all about the range and the specifics of the site, not just an absolute number.

The 'Critical Rail Temperature' is the temperature at which any particular rail gets to a point where the risk of a buckle become 'intolerable' in a risk manegement sense. This does not mean it will buckle - far from it, but just means it's the point we start taking additional precautions.

For new track, the ballast wont be consolidated, no matter how good the wackers and tampers etc were, and it needs to be quite literally shaken down under traffic for full consolidation to be acheived (or something like a Dynamic Track Stabiliser to be used, which speeds things up significantly but brings its own set of issues).

Track gets better at this rate (extract from NR/L2/TRK/001mod014):


There are 4 values we use for CWR:
1) SFT - 'Stress Free Temperature' - The temperature at which, if the CWR rail was cut, it would neither expand nor contract. If works are done overnight and stressing has not been done, this can be very low.
2) CRT(W) - 'Critical Rail Temperature' (see above) where we start taking action. A 'watchman' is usually deployed to site. It's their job to keep a close eye on actual site specific temperatures, monitor the track for any signs of a buckle becoming more likely, and stop traffic if the track becomes unsafe, plus implement and withdraw the higher level measures as tenmperatures rise and fall.
2) CRT(30/60) - A temporary speed restriction of 30/60 is imposed.
3) CRT(20) - A TSR of 20 is imposed.

Different calculations are done depending on the SFT of the rail and the reasons for the problem. A new track where the last ballast train was cancelled in order to avoid a possession over-run will be quite weak, so will have a low SFT.

Here's a hypotetical situation (I know nothing of what's going on Plymouth BTW so this may be nothing like the reasons there):
Track re-laid but things go wrong and possession heavily delayed. Stressing and last ballast train cancelled and thus top ballast quite low and track not fully consolidated, plus low SFT.
SFT= 5degC.
CRT(W)=SFT+15=20degC.
CRT(30/60)=CRT(W)+3=23degC
CRT(20)=CRT(30/60)+5=28degC.

So, it's quite conceivable that a 20mph TSR could be in placed during times when the rail temperature is higher than 28degC, which is warm weather and direct sunlight could be an air temp of only 15-18degC. Hence the current delays.

If the track is not welded up then instead we calculate something called the 'joint closure temperature', which is where all the gaps we've left for expansion close up and the risk of a buckle goes up remarkably, but again, this is subject to a similar calcualtion of CRT values and will result in the same 30/60 or 20mph TSRs.

HTH!

 

[Bald Rick]

Great explanation!

Without trying to complicate matters, there are other factors too - how curved the track is, the type of track sleepers, the type of track restraint (if required), how much and where the ballast is short... all these affect the CRT. what time of year it is (solar gain is obviously more in May / June / July), the prevailing weather conditions, and the track orientation and exposure (ie how and when it is in direct sunlight) all affect the actual rail temperature.

It is quite easy to get a heat speed restriction in February if all the factors are against you (very sub zero installation, no stressing, short of ballast, curved track; on a site exposed to the sun and a warm day subsequently).

When a long spell of hot weather is forecast, it is not unknown to suspend certain types of track work. In some parts of Europe, they just don’t do any in high summer.