For tunnels exceeding 300 m in length, what is the recommended maximum working grade relative to the ruling gradient, in order to control traction effort, drainage, and operational safety?

Introduction / Context:
Long tunnels impose additional resistance on vehicles due to restricted ventilation, safety constraints, and limited opportunities for recovery if stalls occur. To mitigate these effects, the working grade within long tunnels is often set lower than the ruling gradient of the route so that traction requirements, braking, and smoke/heat management remain within safe limits.

Given Data / Assumptions:

• Tunnel length > 300 m triggers a more conservative grade policy.
• Ruling gradient is the maximum grade allowed on the open route for capacity and traction design.
• Question seeks the recommended fraction of the ruling gradient used within long tunnels.

Concept / Approach:

By limiting the tunnel grade to about 60% of the ruling gradient, designers provide margin for emergency stops, vehicle restarts, heat buildup, and drainage control. The reduced gradient also eases ventilation loads by lowering sustained engine power requirements in confined spaces.

Step-by-Step Solution:

Verification / Alternative check:

Design handbooks and alignment guidelines frequently prescribe flatter grades in tunnels for rail and highway facilities, with typical values around 50–60% of ruling gradient depending on systems and safety strategy.

Why Other Options Are Wrong:

• 50%: More conservative than commonly required in many standards.
• 75% or 80%: Too steep for long tunnels; may compromise traction and ventilation.
• 40%: Overly restrictive, potentially uneconomical.

Common Pitfalls:

• Designing tunnel grades equal to ruling gradients, ignoring confined-space constraints.
• Neglecting drainage implications of very flat grades; proper crossfall and channels are still needed.

Final Answer:

60% of the ruling gradient