For a Broad Gauge (B.G.) single-track railway tunnel, what should be the typical clear height above top of rails (TOR) to accommodate rolling stock and services?
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A5.5 m to 5.8 m
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B5.8 m to 6.2 m
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C6.7 m to 7.3 m
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D7.3 m to 7.5 m
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E6.2 m to 6.5 m
Answer
Correct Answer: 6.7 m to 7.3 m
Explanation
Introduction / Context:Tunnel vertical clearance must safely envelope the dynamic outline of trains, overhead equipment (if electrified), ventilation ducts, and maintenance space. For B.G. single track, a clear-height band has emerged from practice to balance safety, equipment needs, and excavation economy.
Given Data / Assumptions:
- Gauge: Broad Gauge (single track).
- Reference: height above top of rails (TOR).
- Assume standard rolling stock and typical service provisions.
Concept / Approach:The 6.7–7.3 m band accommodates train sway, overhead fixtures, and cable/duct allowances without excessive over-excavation. Lower bands may suffice in specific non-electrified contexts, but the stated range is a robust general guideline in planning and exam contexts, particularly where room for ventilation ducts or emergency systems is needed.
Step-by-Step Solution:
Identify TOR as the vertical datum for clearances.Include dynamic envelope, OHE (if applicable), and safety margins.Select the recognized safe band: 6.7 m to 7.3 m above TOR.Verification / Alternative check:
Cross-sections of typical B.G. tunnels with OHE and walkways fit within this envelope while preserving aerodynamic and smoke clearance considerations.Why Other Options Are Wrong:
Bands below 6.7 m risk inadequate space for equipment and safe clearances.Values beyond 7.3 m are conservative and raise excavation costs without generic justification.Common Pitfalls:
Forgetting to check special rolling stock or emergency ventilation requirements that might push toward the upper end of the band.Final Answer:
6.7 m to 7.3 m