Ballast gradation: The best (standard) track ballast contains stones varying in size approximately within which range?

Introduction / Context:
Ballast size influences drainage, interlock, and load distribution beneath sleepers. Railway standards typically specify a well-graded, angular crushed stone with nominal sizes in the few-centimetre range. This question asks for the best (standard) size band commonly quoted in railway engineering texts.

Given Data / Assumptions:

• Mainline track with stone ballast.
• Size range sought is the commonly recommended nominal range for best performance.

Concept / Approach:
Typical guidelines place ballast stone sizes between about 25 mm and 60 mm. This ensures sufficient interlock and drainage while avoiding excessive fouling and ease of tamping. Among the choices, 2.5 cm to 6.0 cm (25–60 mm) matches standard recommendations for mainline ballast gradation.

Step-by-Step Solution:

Verification / Alternative check:
Many manuals and contemporary references cite ~25–60 mm as the preferred range for mainline ballast, balancing interlock and drainage with manageable tamping effort.

Why Other Options Are Wrong:

• 1.5–3.0 cm: Too fine; poor interlock and higher fouling risk.
• 2.0–4.0 cm and 2.0–5.0 cm: Lower bound is small for heavy traffic; less lateral restraint.
• 6.0–10.0 cm: Too coarse; difficult tamping and potential sleeper seating problems.

Common Pitfalls:

• Assuming a single size works best; a well-graded mix around 25–60 mm is preferred.

Final Answer:
2.5 cm to 6.0 cm