Permanent Way Institution - West of England Section Meeting - 28.02.2017 The speaker presented the main parameters that influence the track response to temperature variations and the means to evaluate and control the rail thermal forces. It was discussed the theoretical background and practical elements of managing the track in hot weather for jointed and CWR track, on plain line and S&C. https://www.thepwi.org/calendar/event/view?id=677

1. Constantin Ciobanu
CEng MRes FPWI MCIHT
Principal Track Engineer
West of England
28/02/2017
Behaviour of the track in hot weather
Rail thermal forces for jointed and CWR track

2. The physics of rail thermal expansion
ΔL = α L ΔT
- ΔL is rail extension
- α is the expansion coefficient of the rail steel
- L is the rail length
- ΔT is the rail temperature variation
N = (α E ΔT)·A = σ A
- E is the steel’s elasticity modulus
- A is the area of the rail section
- σ is the rail stress generated by
the temperature variation, ΔT.

3. Track parameters influencing the rail thermal behaviour
• Installation parameters
• Rail type
• Rail temperature
• Track longitudinal resistance
• Track lateral resistance
• Joint minimum and maximum gap - for jointed track
• Joint resistance - for jointed track
• Adjustment switch gap – for CWR

4. Installation parameters
• installation temperature and joint installation gap for jointed track,
• stress free temperature (SFT) for CWR track.
Rail type
N = (α E ΔT)·A = σ A

5. • Rail temperature range: [-14°C, 53°C] (NR/L2/TRK/3011).
Rail temperature (ΔT)
N = (α E ΔT)·A = σ A

6. Joint maximum gap
Gmax = Bf + Df + Dr – 2Db – 2Br

7. Long rail. Short rail

8. Track longitudinal resistance
Three levels of action:
• P1 – resistance due to the friction forces between the rail and
the fastening.
This is usually the first to activate, at very short movements of
the rail. In this case the rail can move through the fastenings and
the sleepers are stationary.
• P2 – resistance due to frictional forces between the fastening
system and the sleeper.
The majority of rail fastenings don’t allow any relative movement
at this level and in this common case P2 is ignored and the
longitudinal resistance is only analysed for movements at the
other two levels.
• P3 – resistance due to frictional and passive resistance forces
between the sleeper and the ballast. In this case the rails and
sleepers move together relative to the ballast.
This resistance is typically in the range of 6 (tamped) to 10
(consolidated) kN/sleeper.

9. Track longitudinal resistance – old vs new
Old track components
BR2 baseplate with Macbeth spring spike anchors
Bullhead rail Panlock chair fastening
P1 ≈ 0
P1 > P3
Pandrol Fastclip
Pandrol ZLR – Zero Longitudinal Resistance
P1 = 0
Modern track components

10. Track longitudinal resistance

11. Track lateral resistance
Three levels of action:
• Sleeper bottom (30-50%)
• Sleeper sides (40-60%)
• Sleeper end (10-30%)

12. Methods to increase the track lateral resistance
• Dynamic track stabilisation (DTS)
• Increase ballast shoulder dimensions
• Lateral resistance plates
• Glue ballast
• Heavier/special sleepers

13. Joint resistance force
The fishplated rail expansion joint has two
main functions:
• to maintain the alignment of the rail running
surface.
• to reduce the rail thermal forces by allowing
rail expansion or contraction.
R = 4 n N f

14. Jointed track response to temperature variations

15. Jointed track response to temperature variations
… the detailed calculation process will be presented
in the PWI Journal – Vol 135 Part 2 – April 2017

16. The track resistance forces retain thermal
forces in the rails and define two
envelope branches of the joint gap loop:
- 12-13 - compression force
- 7-8 - tension force
Significant difference compared to the
free thermal expansion model.
Delayed response of the track to rail
temperature variations:
A one day temperature loop is contained
in the envelope loop.
Morning : 15°C, joint gap of 9 mm.
The temperature increases during the day
to a maximum of 35°C ,joint gap is
reduced to 4 mm.
During the night the temperature
decreases to 10°C , gap increases to 6.3
mm.
The next morning the temperature
reaches 15°C, the gap remains 6.3 mm
because the resistance forces have not
been reversed during the 5°C increase
from the minimum 10°C reached during
the night.

17. • At installation the thermal forces through the length of the rail are
consistently null.
• This state will never return naturally throughout the service life of the
track. The thermal forces will never be consistently equal through the
entire length of the rail, unless joint gap resetting or any similar
maintenance works are undertaken.
• From thermal perspective the track behaves as a hysteretic model
where the current state of the internal thermal forces is dependent on
the rail temperature history.

18. Continuous Welded Rail (CWR)
The track composed of long rails which develops a central immobile (fixed) zone,
where no rail movement due to temperature variation occur. (UIC definition)
Lb – Stress transition length (Breathing length) ≈90 -120 m. CWR L > 200m.
Shorter lengths of track can be considered CWR from maintenance perspective.
L > 37 m NR/L2/TRK/3011 (2012) and L > 30 m NR/L2/TRK/2102 (2016)

19. Stress transition zone – one day temperature variation

20. Stress transition zone
Rail temperature difference
• Tunnel (covered track) to natural sunlight track
• Significant changes in sunlight exposure – passage from cutting to
embankment, changes in the direction of the track
• Passage over a river – condensing water will reduce the rail temperature
compared to the track over embankment
• Closure weld / stressing procedure (heat influence zone, different SFT)

21. Stress transition zone
Track structure variation
• Presence of S&C
• Change in rail type
• Track over the mobile bearing of a bridge

22. S&C – thermal forces
• 2 = 4 ?
• Switch rails are free to expand
• Point operating equipment allowed to switch tracks

23. S&C – thermal forces. 2 = 4
• Stress transfer block – closure rail transfers the stress fully to the stock rail.
CR/SR tied together
• Switch/Stock rail thermal interaction devices (ball & claw or similar).
CRs have limited independent expansion.
Creep monitor. It is not a monitoring device but a partial stress transfer device.
• POE designed to fully allow the switch rail expansion . CRs expand freely relative to SRs

24. Ball and Claw - Switch/Stock rail thermal interaction devices

25. Adjustment Switches
Joints with overlapping rail ends, allowing longitudinal rail movement and so
dissipating thermal forces when CWR abuts jointed track or other features not
designed to withstand thermal forces.
• CWR to jointed track
• CWR to CWR (when the track structure
changes)
• Over the mobile bearing of long bridges
UIC 774 R. Track Bridge Interaction

26. Adjustment Switches
Joints with overlapping rail ends, allowing longitudinal rail movement and so
dissipating thermal forces when CWR abuts jointed track or other features not
designed to withstand thermal forces.

27. Adjustment Switch – rail breathing: CWR to Jointed Track (18.288m rails)
theoretical calculation

28. Adjustment Switch – rail breathing: CWR to Jointed Track (9.144m rails)
theoretical calculation

29. Adjustment Switch – rail breathing: CWR to CWR
Special design might be required, especially if one CWR section is over a long bridge. (UIC 774 R. Track Bridge Interaction)
theoretical calculation

30. Track buckling

31. Track buckling
The track buckling has two main stages:
• Trigger phase (A-B) – track reaches unstable equilibrium
• Energy release phase (B-C) – track releases tension and assume a new
stable equilibrium state

32. Track buckling (UIC 720R)
The buckling triggering energy is evaluated and a temperature limit is established,
dependant on required factor of safety.
Definition of the Critical Rail Temperature (CRT) – a safe rail temperature increase.

33. CRT – Critical Rail Temperature
NR/L2/TRK/001 Module 14. Inspection and Maintenance of Permanent Way. Managing Track in Hot Weather.
The rules for evaluating the CRT are based on buckling calculations.