Case Studies

Case studies illustrate the unique experience behind our services.

Measurement of track deflection and assessment of track stiffness effects on track performance provide unique insight to track load distribution and track geometric stability.  The most common issues develop with soft track support and low track stiffness.

This case study highlights the effect of very high track stiffness, low track deflection on track geometry and track structural performance.  The high stiffness limited the ability of the track to distribute loads appropriately, concentrating load on the tie below the railroad wheel.  The reduce load distribution caused high tie-ballast contact stress leading to tie bottom abrasion. 

A forensic analysis could not identify a single factor leading to reduced service life of some generations of specific concrete ties.   A review of the locations where tie performance problems developed highlighted the potential for track settlement and the formation of ballast gaps.  A review of the impact of the tie-ballast gap on tie performance was conducted including a structural dynamic assessment of changes in tie dynamic response.  The analysis included bending and vibration mode frequency shifts that highlighted a vibration mode shift when the tie changed from supported by ballast to hanging above the ballast.   

A site investigation and measurement campaign to assess the relative contribution of damage from freight and passenger traffic was undertaken.  The specific test sites served as alignment areas where data from systemwide measurement of track geometry, track deflection, and ground penetrating radar were used to assess zones of similar track performance.  For each zone, the relative degradation was used to assess maintenance tamping intervals to develop a basis for assessing expected increases in maintenance with increased freight axle loads. 

Sites with large vertical deflection due to loss of track vertical support were assessed to document track and site conditions as well as effects on the track superstructure.  In the photo below, the track vehicle is loading the track and the effect of the track transition is clearly visible in the image which reflects the deflection variation across the change in stiffness.  The lifted spikes are one sign of the stiffness change effects on the track superstructure. 

Track maintenance effects the stability of track mainly by changing the ballast structure and tie-ballast friction available to resist track buckling loads.  The structure of heavily interlocked ballast is broken down during surfacing as the track position is aligned to correct track settlement and track geometry deviations.  The rail neutral temperature is also impacted by maintenance.  Combined, both the change in rail neutral temperature and the lateral resistance changes can impact track safety making it critical to reestablish track lateral resistance by compacting and consolidating ballast.  In this study, the track lateral resistance before and after surfacing was measured.  Then, various track stabilization methods were compared including dynamic track stabilizer and application of traffic.  

A test track section was construction at the Transportation Technology Center in Pueblo, CO to assess constructability factors for direct fixation track designed to handle heavy freight traffic constructed to demanding high-speed rail tolerances.  The track was subject to heavy freight service for over 10 years providing a solid basis for assessing the practicality and economics of direct fixation track.  Two forms of direct fixation track were tested including a dual block track and a direct fixation fastener system, shown in the photo below.  Both commercially available systems were intended for heavy freight service and both provided significant resilience.