Composite Sleeper Index (CSI): What does this index primarily indicate in sleeper selection and procurement?

Introduction / Context:
Timber sleepers vary in species, density, strength, and durability. The Composite Sleeper Index (CSI) combines multiple attributes into a single figure to assess and compare the suitability of wooden sleepers for track service.

Given Data / Assumptions:

• CSI aggregates mechanical and durability characteristics.
• Used by railways to evaluate timber sleeper options for procurement.

Concept / Approach:
CSI is a weighted index derived from parameters such as specific gravity, modulus of elasticity, crushing strength, and resistance to decay/borers. A higher CSI generally correlates with longer expected service life and better performance under traffic loads and environmental exposure.

Step-by-Step Solution:
Identify performance drivers: strength, stiffness, durability.Combine into an index to rank timber types objectively.Use CSI threshold values to approve or reject sleeper lots.

Verification / Alternative check:
Procurement specifications often stipulate minimum CSI and preservative treatment standards. Field performance audits validate that higher CSI timbers perform better in track.

Why Other Options Are Wrong:
Number of sleepers per rail length: Determined by sleeper density policy, not CSI.Permissible stresses in steel sleepers: Material-specific design data, unrelated to CSI for timber.

Common Pitfalls:

• Treating CSI as a sole criterion while ignoring treatment quality and seasoning.
• Using CSI values without considering route class and traffic density.

Final Answer:
Suitability of wooden sleepers (quality assessment for service)