The best heirloom must surely be one you can use and display regularly. KiwiRail does this every day, putting on display some of New Zealand’s most striking scenery as the Overlander Passenger Rail service traverses between Wellington and Auckland. The early pioneers carved out these vistas in the late 1800s and early 20th century, linking the two cities with a band of steel.

Still going strong 130 years on, it’s a great example of supreme resilience – but future trends in rail present some interesting challenges. Firstly, we are seeing an ever-increasing patronage in urban passenger rail and a high demand for reliable, safe trains that run on time. For example, the original lines in metropolitan Auckland now carry a train every 10 minutes on average. 
In addition, freight trains are now expected to carry larger, heavier loads. Originally at 10-12T/axle, they are now at 18-22T/axle and this is likely to increase to 25T/axle. Can the century-old infrastructure cope?

Extending the Life of What Lies Beneath

Meeting these market demands has driven modernisation in many aspects of rail – coaches/wagons, suspension systems, locomotives, rails, sleepers and fastenings have all evolved and improved. But when the formation – or what lies beneath the ballast – is at fault, lasting effective improvements are hard won. 

Due to age and historic constraints, track issues are increasing. The challenge is to achieve greater resilience in the old formations or to face network collapse. WSP has been working with KiwiRail to develop a systematic approach to evaluate formation issues, then select and implement the necessary repairs.
The back-to-basics approach looked at the issues and what needed to be resolved, then identified the minimum intervention required and the level of guidance field staff would need. 

Revisiting the Past to Improve the Future

WSP looked at six case studies and checked the factors influencing track formation issues. Previously unrecognised influences were identified, including both direct factors and extraneous factors. Common elements emerged when looking at each influencing factor from a first principles approach. 

Some simplistic past conclusions were found to be based on incorrect assumptions. So a practical method of investigation was developed and achievable repairs covering most situations were conceived and established. 

As part of this exercise, a number of repairs have been executed. At one problem site, almost midway between Wellington and Auckland where poor formation conditions led to a derailment, a sand blanket repair was constructed. It has required no subsequent work or intervention for over three years, which is a good outcome. 

Sufficient confidence in the process led to the development of a Track Formation Standard and two Task Instructions. These cover the remediation (construction) of problem formations and the investigation of formation issues through a rational, reasoned approach.

A Track Failure Snapshot

There are many contributing factors to track failure. Put simply, when the track is under train load the hard ballast pulverises the formation and, if there is any free water present, the powdery residue forms a mud slurry. The dynamic loading then pumps this sticky mud into the ballast, clogging it up. 

The result is a loss of line and/or level. The track may appear wavy and there may be excess rail movement accompanying wheel passage. This movement may be concentrated on one side of the track causing a twisting movement. In extreme cases, this is the root cause of derailments – and both the prime and consequential financial implications are huge.
The challenge is to achieve greater resilience in the old formations or to face network collapse.