Indian Railways (B.G. trunk lines): What is the prescribed minimum depth of stone ballast under sleepers for broad-gauge trunk routes?

Introduction / Context:
Minimum ballast depth is a key design and maintenance parameter that ensures adequate load distribution, drainage, and tamping space beneath sleepers. Indian Railways specifies higher minimums for trunk lines due to higher speeds and axle loads.

Given Data / Assumptions:

• Track: Broad Gauge (B.G.) trunk route.
• Ballast: Crushed stone ballast assumed.
• Question seeks the standard minimum depth under sleepers for trunk lines.

Concept / Approach:
Heavier traffic corridors require larger ballast cushions. Typical practice prescribes about 30 cm minimum for B.G. trunk lines, allowing room for stress distribution and maintenance tamping without punching into subgrade.

Step-by-Step Solution:

Verification / Alternative check:
Exam guides and practice manuals for Indian Railways list 30 cm as the minimum for B.G. trunk lines, with higher values in special cases (high-speed or heavy axle load corridors).

Why Other Options Are Wrong:

• 15 or 20 cm: Too shallow for trunk lines; risks ballast breakdown and subgrade stress.
• 25 cm: Used on lighter routes; below trunk-line standard.
• 35 cm: Not the minimum; sometimes used as enhanced ballast in demanding conditions.

Common Pitfalls:

• Confusing average ballast depth with minimum prescribed depth.

Final Answer:
30 cm