10 - Dealing with Climate Extremes

Question 1

Effects of extreme climate on railways

Answer 1

Grounds movement and poor track alignment on dry or saturated ground
Tracks buckling
Overhead lines sagging and damaged
Flood damage and landslides
Uncomfortable passengers
Air-con itself generated heat (bad in underground systems)
Coach design must maintain 21 degrees Celsius

Question 2

Track - stress relief

Answer 2

To reduce longitudinal rail stress, expansion joints are included
Older bolted track includes small gaps at each joint
Action depends on ability of track to move (fastening system design)
Only a certain range of movement can be accommodated
If stress isn’t kept low enough, rail buckling is likely in hot weather

Question 3

When does rail buckling happen?

Answer 3

Rarely spontaneously
Often triggered by an approaching train
Vertical rail deflection lifts track, reducing its resistance to lateral shift
Cold weather reverses problems - high tensile stress and rail breaks

Question 4

Network Rail standard for managing track in hot weather

Answer 4

> 36C air temperature
Limit freight to 45mph
Limit passenger vehicles to 90mph

Question 5

Characteristic buckling methods

Answer 5

Symmetric mode (1 half wave)
Antisymmetric mode
Symmetric mode (3 half waves)

Question 6

Detailed modelling of track

Answer 6

Idealise rail as two rigid bars
Elasticity is concentrated in a rotational spring
With section aligned, spring is unstressed, bars compressed
If sideways disturbance happens at one end - spring acts to restore alignment, but axial force acts to make deflection longer
Transition from stable recovery to buckle is at a critical axial load (i.e. a critical temperature)

Question 7

Detailed look at buckling

Answer 7

Models are very complex
Everything is for two rails, sleepers and ballast, not just a simple beam
Rail-sleeper fastenings add torsional stiffness
Sleepers have non-linear frictional resistance moving over ballast

Question 8

Results of track buckling model

Answer 8

Showed there is a ‘critical point’ corresponding to a critical force/temperature for a given lateral displacement

Question 9

Critical point for rail buckling

Answer 9

Critical point is the minimum temperature rise above neutral at which buckling can happen
Below this the track is stable
Above this some higher temperatures can be tolerated if there’s little misalignment, but this is an unstable situation
If misalignment reaches limit, track will rapidly shift to buckled geometry

Question 10

What does critical temperature rise for?

Answer 10

Higher lateral resistance: better quality ballast; shoulders of ballast alongside track; stiffer rail fastenings; stiffer sleepers
Reduction of axial force: expansion joints; higher stress free temperature

Question 11

Overhead line electrification

Answer 11

Overhead line contact wire - copper or copper based alloy
Conductivity of 1.7x10^-5 /C
Overhead line for higher speed is typically a catenary and contact wire
Trams and sidings have a single wire system with shorter span lengths
Fixed termination - tensioned and fixed in place
Automatic tensioning - weights or spring tensioners

Question 12

What happens when overhead lines go wrong?

Answer 12

Bow de-wirement
£1.2 million delay cost
38 hours to repair with 6 lines closed
Catastrophic and complex failure modes

Question 13

Why do problems with overhead lines cause so much damage?

Answer 13

If the pantograph ‘dewires’ and rises up, it hits and destroys the overhead line for a long distance
Wires may also be interdependent, so failure on one line may affect several others