Rail fastenings are just one part of the track structure, but they perform a number of critical functions. It is widely understood, of course, that one of their principle functions is to restrain the rail, longitudinally i.e. to control rail creep. What is less well understood is that the amount of restraint that is required varies from one application to another, and depends on many factors some of which depend on the detail design of the rail fastening, and some of which do not.
There are four main reasons to control rail creep:
• To constrain continuous welded rail when it is subjected to temperature variations.
• To prevent movement of the rail when it is loaded longitudinally by traction and braking forces.
• To protect switches and crossings, and insulated joints, from excessive longitudinal forces
• To limit longitudinal forces transferred between the track and other structures such as bridge decks.
In general, the first three of these involve restricting rail creep, but the last may involve allowing the rail to slip.
When continuous welded rail (CWR) was first introduced, it was generally assumed that the ideal rail fastening system would fix the rail to the tie as rigidly as possible, and withstand the highest possible force without permitting any movement of the rail, relative to the tie. This approach led to target values being set for longitudinal restraint tests on rail anchors and on elastic rail fasteners. In reality, it has always been necessary to make some compromises to allow for the requirement for other track parameters, such as the requirement for elasticity in tie pads, and the fact that the resistance of the tie in the ballast is also finite. Today, we know that there are many thousands of miles of track in North America, with CWR, functioning very well even though the creep resistance is less than that prescribed for new fastenings, but we also know that real problems can occur when the rail slips in an uncontrolled way. The time is right for a fundamental review of rail longitudinal restraint issues, so that appropriate limits can be set for type approval of new fastening systems and for maintenance of existing track.