Track curvature correction: Rails are bent (curved) in the yard before laying when the degree of curve exceeds approximately what value?

Introduction / Context:
On sharper curves, straight rails resist seating properly and develop undesirable lateral forces and gauge variations during traffic. To achieve a true circular arc and smooth running, rails are pre-bent (curved) before installation once the curvature exceeds a practical threshold. The question asks for the typical degree of curve beyond which pre-bending is adopted.

Given Data / Assumptions:

• Degree of curve is defined in standard railway geometry (angle subtended by a 30.5 m chord in many systems).
• We assume mainline practice where pre-bending is used for sharper curves to minimize joint batter and lateral creep.

Concept / Approach:
Experience shows that when curves are sharper than a few degrees, field bending ensures proper contact and reduces maintenance. Conventional exam standards and handbooks often quote about 4° as the changeover point for pre-bending.

Step-by-Step Solution:

Verification / Alternative check:
Track geometry manuals and typical MCQ references cite ~4° as the practical threshold beyond which rails are bent prior to laying to match curve radius and reduce kinks at joints/welds.

Why Other Options Are Wrong:

• 1°, 2°, 3°: Curves are too mild to mandate pre-bending in most practices.
• 6°: Too stringent; pre-bending is already recommended on curves sharper than ~4°.

Common Pitfalls:

• Confusing mandatory pre-bending with on-site easing; mild curves may be adjusted without full bending.
• Mixing degree-of-curve conventions (ensure consistent definition across regions).

Final Answer:
4°