Ballast grading for flat-bottom rail sleepers: What is the maximum size of ballast recommended for flat-bottom sleeper tracks?

Introduction / Context:
Ballast grading influences drainage, load distribution, track resilience, and maintenance. The maximum nominal size must be selected to prevent fouling while ensuring interlock and ease of tamping around flat-bottom sleeper seats.

Given Data / Assumptions:

• Standard track with flat-bottom sleepers (timber/concrete/steel bearer types supporting flat-footed rails).
• Question targets the upper size limit of ballast grading.

Concept / Approach:
Railway specifications commonly use coarse ballast up to a nominal maximum of around 50 mm for main lines. This offers good interlock and drainage while avoiding oversize that can hinder compaction or damage components. Smaller maxima (e.g., 40 or 33 mm) may be used in special cases but 50 mm is the typical main-line value presented in standard questions.

Step-by-Step Solution:

Verification / Alternative check:
Permanent way specifications across several administrations align around a 50 mm top size for mainline tracks using flat-bottom sleepers.

Why Other Options Are Wrong:

• 40, 33, 25, 20 mm: Finer gradings; may be used for specific cases or secondary lines, but not typically the mainline maximum referenced in standard exam practice.

Common Pitfalls:

• Confusing nominal maximum size with average particle size or grading envelope limits.

Final Answer:
50 mm