Thermal behavior of rails: When rails move out of position due to insufficient expansion gaps on hot days, the phenomenon is called:

Introduction / Context:
Rails expand with temperature. If expansion is restrained (inadequate gaps or high resistance), compressive forces build up and the track may buckle laterally, risking derailment. Recognizing this failure mode is essential for safe operations in hot climates.

Given Data / Assumptions:

• Jointed or long-welded rail with insufficient allowance for thermal expansion.
• High rail temperatures causing compressive stresses.

Concept / Approach:
Thermal expansion without adequate relief creates axial compression. When the critical compressive load exceeds track lateral resistance, the rail alignment kinks—termed buckling or sun kink. Proper neutral temperature management and ballast resistance mitigate this risk.

Step-by-Step Solution:
Identify root cause: insufficient expansion gap or high locked-in temperature.Understand mechanism: compression leads to lateral instability (buckling).Name the phenomenon: buckling (sun kink).

Verification / Alternative check:
Inspection records often note lateral kinks during heat waves where expansion gaps were inadequate or anchors ineffective.

Why Other Options Are Wrong:

• Creeping: slow longitudinal movement due to traffic forces, not sudden lateral instability.
• Hogging: vertical distortion usually at joints due to differential settlement/battering.
• None/Lipping: do not describe this thermal lateral deflection.

Common Pitfalls:

• Confusing creep with thermal buckling; one is gradual longitudinal movement, the other is sudden lateral instability.

Final Answer:
Buckling (sun kink).