Rails Longitudinal Movement Explained

The question asks about the specific term for the longitudinal movement of rails relative to the sleepers in a railway track. This phenomenon is a common occurrence in railway engineering and has a distinct name. Understanding this movement is crucial for maintaining track stability and safety.

Creep of Rails Definition

What is Creep?

Creep of rails is defined as the longitudinal (along the length) movement of the rail in a railway track, specifically with respect to the sleepers. This movement often occurs in the direction of traffic, but it can also occur against it, or unevenly.

• It is an inherent and unavoidable characteristic of railway tracks, varying in magnitude depending on several factors.
• Controlling and minimizing rail creep is essential for efficient and safe railway operations.

Causes of Creep of Rails

The longitudinal movement of rails, or creep, is caused by several factors acting individually or in combination:

• Wave Action: As a train wheel rolls over the rail, it creates a vertical deflection, forming a wave ahead of the wheel. This wave tends to push the rail forward in the direction of traffic.
• Braking and Acceleration Forces: When trains apply brakes, they exert forces that tend to push the rails backward. Conversely, during acceleration, forces are exerted that push the rails forward.
• Thermal Expansion and Contraction: Changes in temperature cause rails to expand and contract. If not properly accounted for with expansion joints or managed in continuous welded rails (CWR), these forces can induce significant longitudinal movement.
• Percussion of Wheels: The impact of train wheels, especially at rail joints, can contribute to a slight forward movement of the rail over time.
• Starting and Stopping of Trains: Sections of track where trains frequently start and stop experience more pronounced creep due to the dynamic forces involved.

Effects of Creep on Rails and Track

Uncontrolled creep of rails can lead to several detrimental effects on the entire track structure:

• Sleeper Skewing: Sleepers (ties) can get pushed out of their square position relative to the rails, leading to misalignment.
• Gauge Variation: The track gauge (the distance between the two rails) can become irregular, leading to unsafe running conditions and potential derailments.
• Buckling of Track: In severe cases, especially combined with high temperatures, excessive creep can cause the track to buckle or kink, making it impassable.
• Distortion of Points and Crossings: Creep can cause serious misalignment and operational issues at critical track components like switches (points) and crossings, potentially leading to accidents.
• Damage to Fastenings: The fasteners (e.g., clips, spikes) holding the rail to the sleeper can become overstressed or damaged, reducing track integrity.

Controlling Creep of Rails

To mitigate the adverse effects of the longitudinal movement of rails, various measures are employed in track construction and maintenance:

• Anti-Creepers (Rail Anchors): These are devices clamped to the rail and bearing against the sleeper to prevent the longitudinal movement of the rail relative to the sleeper.
• Adequate Ballast Resistance: A well-packed and stable ballast bed provides strong resistance against the movement of sleepers, which indirectly helps control rail creep.
• Continuous Welded Rails (CWR): By eliminating most rail joints, CWR significantly reduces points where creep typically initiates and propagates.
• Proper Rail Laying Practices: Ensuring correct rail expansion gap management and tensioning during installation helps in managing thermal stresses and limiting creep.

Rails Terminology Explained

It's important to differentiate creep of rails from other terms related to railway tracks:

• Coning of Wheels: This refers to the conical shape given to the treads of railway wheels. Its purpose is to allow the wheels to self-centre on the track and negotiate curves smoothly, not a movement of the rail itself.
• Bending of Rails: This describes the vertical deflection or deformation of the rail under the weight of a train. While rails do bend vertically, it is not the longitudinal movement described in the question.
• Hogged Rails: This term refers to a specific type of deformation where the ends of the rails at joints get bent downwards, typically due to repeated heavy impacts from rolling stock. It's a localized deformation, not a general longitudinal movement of the entire rail section along the track.