Statement: A major railway accident involving a mail train was averted due to the presence of mind of a signalman at a wayside cabin.\nCourses of Action:\nI. Clear 50 km of track ahead of every mail train, keeping it traffic-free.\nII. Immediately make all railway signalling fully automatic.\nIII. Reward the signalman to encourage such alertness.

Introduction / Context:
A near-miss underscores human vigilance within layered safety. Rational actions are those that reinforce effective behaviors and adopt feasible improvements without imposing unrealistic blanket rules.

Given Data / Assumptions:

• Incident was prevented by a vigilant signalman.
• Network capacity is high; tracks are shared and scheduled tightly.
• Automation upgrades take time, investment, and testing.

Concept / Approach:
I (keep 50 km traffic-free ahead of every mail train) is operationally infeasible and would cripple throughput. II (immediately make signalling fully automatic) is desirable long-term but cannot be done “immediately” across a vast network; staged upgrades are realistic. III (reward) is proportionate and builds a culture of safety.

Step-by-Step Solution:
1) Commend and reward the signalman; share the case as a learning note.2) Audit that section: interlocking, fail-safes, communication protocols; plan targeted automation upgrades.3) Refresher training stressing vigilance and escalation.

Verification / Alternative check:
Recognition improves morale and attentiveness across staff, while measured tech upgrades address systemic risks.

Why Other Options Are Wrong:
I: Logistically impossible; harms service. II: Unrealistic immediacy.

Common Pitfalls:
Knee-jerk blanket rules; underfunded, rushed automation.

Final Answer:
Only III follows.