Ballast sizing for metal sleepers with rounded edges For permanent-way using metal (steel) sleepers that have rounded rail-seat edges, what is the recommended maximum nominal size of ballast to avoid cutting and to maintain good interlock?

Introduction / Context:
Ballast grading and maximum particle size must suit the sleeper type and seat geometry. Metal sleepers with rounded edges tolerate slightly larger particles than sharp-edged seats because point-contact cutting is reduced. Choosing the correct size balances drainage, load distribution, and maintenance effort.

Given Data / Assumptions:

• Track deploys metal sleepers with rounded contact edges at rail seats.
• Ballast is angular, clean, and well-graded.
• Objective: minimize sleeper damage while ensuring interlock and drainage.

Concept / Approach:
Ballast size affects (1) contact stresses at sleeper interfaces, (2) packing stability, and (3) permeability. With rounded edges, the risk of cutting is lower; a 50 mm nominal maximum size often provides robust interlock and drainage while reducing the fine generation that would occur with undersized ballast under heavy traffic.

Step-by-Step Solution:
Assess sleeper-seat edge: rounded edges reduce stress concentration.Select a size that maintains interlock yet avoids excessive point loading.Industry practice recommends up to about 50 mm nominal size in this case.

Verification / Alternative check:
Track standards frequently specify 50 mm nominal maximum for mainline ballast with metal sleepers, confirmed by performance data on settlement and abrasion.

Why Other Options Are Wrong:
30 mm or 20 mm: Too small for mainline stability; tends to break down and clog.40 mm: Acceptable in some contexts but 50 mm is the common upper nominal for rounded metal seats.25 cm: Dimensionally incorrect (far too large).

Common Pitfalls:

• Selecting oversize ballast for sharp-edged seats, which promotes cutting and sleeper damage.
• Ignoring cleanliness—fines buildup undermines drainage regardless of nominal size.

Final Answer:
50 mm