investigating neutral stress free temperature



Question ?

Does The Rail SFT change during the change of daily temp's


OBSERVATIONS : Whilst working on the FMG line .


Observing the FMG newly laid line Solomon line which starts at 175kms and runs up to 300 kms to the Firetail Iron ore mining area .


The rail line is being used to to move 230 wagons a day with some 140 tons per wagon by 3 locos . They allow one load a day only until the passing lanes are ballasted .


There are several passing lanes still in their skeleton form meaning that the ballast is missing .     The passing rail line is connected to the main line and runs for some plus 2.5kms .    The new line is fully ballasted but has not yet consolidated .


The SFT measurements were carried out from early morning before sunrise . Rail temps were in the 23 to 25C range before sunrise .       Once the sun had risen and was shinning on the track the temps reached up to 54C degrees .


The method used to measure the SFT was with the dStresen system of Jury & Jury Technologies Ltd N.Z.     It was found that the SFT at the line temp of 25C was in this case 16.5 & 18C  East & West Rail .

Latter in the day this rail with the temp now sitting at 54C had SFT's ranging from 33C to 49C .    The following morning we would revisit  these locations again at rail temps in the 23 to 25C range , would again have SFT at 16 to 18C .


The dStresen system was indicating to us the the rail line had been ballasted at these low temps and this theory was put to McDow engineers and our observation was correct .

The rail in its skeleton form was clipped up at higher temps and the McDow engineers thinking was that the skeleton track would hold its higher SFT position that they had clipped it up at .

The dStresen team stopped at a skeleton track to study the track with out the ballast .

It was very easy to observe the scuff marks the sleepers leave from the coolest time to the warmest time of the day .   The tangent track showed only a small movement of 10 to 12 mm between each sleeper .      On the curves the movement pushing the curve outwards was about half this distance 5 to6 mm .


This rail is connected to the main track through the switch and turnout system .

The SFT of this passing lane needs very little compressive load to push out the corners and the scuff marks were very uniform up & down the rail line . 


Back to the new main line .

The only difference is that the ballasted line needed to form some force before the rail SFT started to follow the line temp  .   I noted that the force needed to move the SFT ranges between 5 to 15 tons per rail .

The Numa line 164 to 175 kms was measured many times at different times of the day .


To find out what the true SFT is on these newly ballasted lines that spend all day in compression when the summer temps take hold , is to take a run down the line several hrs before the Sun hits the rail , we have a range of 16 to 18C , SFT .

McDow eng- then decided to bring in the crews to ajust the line .    The temperature before cutting the rail was 36C in the evening and slowly cooling .


The rail was in a small amount of compression and once this was cut out a further 18mm was taken off to readjust the SFT to 40C .


2 days later the dStresen crew went back to this site and remeasured .  The SFT had barely changed still under 20C .

Why was this phenomenon happening ?   We were told that the adjustments were done by the book . The problem is to do with the changing SFT .

When the line is cooling down the sleeper unloads itself from the ballast and reverses itself until the sleeper makes solid contact with the ballast on the other side .  In the process of doing this the line temp and the SFT are very close to each other .


There is only one way one way of dealing with adjustments of the SFT on lines that are moving and this is to adjust very early in the morning when the line temps have fallen to or near their lowest temperatures .   This way the rail line is generating its maximum force and is very easy to get correct numbers on how much rail needs to be cut out to end up with the final SFT .


The dStresen crew worked out from their figures that some 250 mm should have been cut out of the rail to end up with the correct SFT of 40C or higher .


Since then it was discovered that the dStresen information was correct . There was a atempt to put the verse on the rail in the morning before the Sun showed itself .





All rail lines move to some degree .


Rail lines that have old ballast will move their SFT on a regular basis because the ballast has lost its ability to lock the rail line in place .

Ballast is the key to controlling the stability of all continuously welded rail lines .


Mr Ian Marks says that a train moving at 70 kmph needs a million tons of freight before the ballast becomes consolidated .  

 Consolidation will happen if the chipped rock is capable of locking together to form the necessary friction to prevent  the SFT changing .


Laying ballast first thing in the morning is not a good idea if the temps are low .


Spreading the ballast later in the day when temps are in a favorable position and are slow on the cooling phase giving plenty of time to carry out this part of the operation is recommended .

The tamper crews would have plenty of work to do first thing the following morning to keep up production .