INTRO: Replacing single sleepers with closely spaced double-H prestressed concrete frames provides better support for the rails, while their stiffness allows eddy current braking by eliminating lateral buckling
BYLINE: Prof Dipl-Ing Dr techn Klaus Riessberger
Technical University of Graz
TRADITIONAL BALLASTED track has developed over many decades, displacing more sophisticated early designs owing to its simplicity and low cost. Research has tried to explain how it works, but to this day not all functions are clearly understood. However, the role of ballast in terms of resilience for load distribution and lateral resistance for buckling safety is quite evident.
Attempts have been made to strengthen ballasted track to withstand higher demands. Sleepers grew longer and heavier, ballast cross-sections became larger, while the technology of laying and maintenance was refined.
Some new ideas materialised such as frame track support (Soviet Union, Austria), and more recently wide sleepers placed side by side (Germany). The Japanese ladder-track tries to combine the advantages of an innovative structure with flexibility in vertical adjustment.
Another line of thinking produced solid track designed along civil engineering principles without ballast (Germany, Japan, and many tests elsewhere). Not surprisingly, the cost is much higher than traditional track.
Sleepered track laid in stone ballast has been substantially improved, notably through the introduction of continuously-welded rail on concrete sleepers with elastic fastenings, along with an increase in the quantity and quality of ballast.
On-going research has sought further improvement by trying to explain the interaction of forces, stresses and deformations. With higher speeds and axleloads, a number of shortcomings persist, particularly on lines carrying high-speed trains: