Extra ballast shoulder on curves: exemption threshold An extra ballast shoulder of 7.5 cm is typically provided on the outer side of curved track for stability. Up to which degree of curve is this extra width generally not provided (i.e., exemption applies)?

Introduction / Context:
On curves, centrifugal effects and lateral forces demand better lateral resistance from ballast. Hence, extra ballast shoulder is provided on the outer side to enhance stability. However, on very gentle curves the lateral demand is low and standards may exempt additional width.

Given Data / Assumptions:

• Outer shoulder extra width considered: 7.5 cm.
• We refer to typical mainline standards for ballast shoulders on curves.
• Degree of curve (D) relates inversely to curve radius (gentler curve → smaller D).

Concept / Approach:
For small D (gentle curvature), rail-seat lateral loads are close to those on tangents. The stability from normal shoulder is adequate, so the 7.5 cm extra is not mandated. A commonly cited cut-off is around 2°, beyond which extra shoulder is specified progressively for sharper curves.

Step-by-Step Solution:
Relate curve sharpness to lateral force: sharper curve → higher lateral force.Identify exemption threshold: very gentle curves (about 2°) need no extra shoulder.Therefore, extra 7.5 cm is not provided for D = 2°.

Verification / Alternative check:
Track maintenance guidelines tabulate shoulder requirements versus curve degree; exemptions at the gentlest curvature confirm this rule-of-thumb.

Why Other Options Are Wrong:
3°–6°: These are sharper curves where extra shoulder is generally recommended to resist lateral drift and maintain line.

Common Pitfalls:

• Applying the exemption to sharply curved turnouts where higher lateral forces exist.
• Neglecting local conditions (weak formation, poor drainage) that may still require extra ballast even on gentle curves.

Final Answer:
2°