Select the typical cross-sectional form adopted for railway tunnels to balance construction practicality, structural action, and loading envelopes.

Introduction / Context:
Railway tunnels must accommodate track, clearances, and dynamic envelopes while providing an efficient structural shape against earth and water pressures. The chosen cross-section affects lining economy, aerodynamic performance, and ease of construction. Traditional railway tunnels often adopt compound-curve (polycentric) profiles to meet these requirements.

Given Data / Assumptions:

• The options include geometries commonly referenced in tunnel design.
• Loading considerations include overburden pressures and live load envelopes.
• We focus on conventional single-track or double-track railway tunnels in rock or soil with linings.

Concept / Approach:

A polycentric (compound) arch uses more than one center to shape the intrados/extrados, creating a horseshoe-like outline. This profile combines a curved crown for arching action with near-vertical sidewalls for clearance efficiency, achieving a balance between structural behavior and usable space.

Step-by-Step Solution:

Verification / Alternative check:

Historic masonry, cast-in-place concrete, and modern segmental linings for railways frequently use horseshoe/polycentric shapes. Circular sections are more common in pressure-balanced TBM drives; rectangular sections suit cut-and-cover. Parabolic is less typical for full perimeter linings.

Why Other Options Are Wrong:

• Rectangular: Structurally less efficient for uniform ground pressure; stress concentrations at corners.
• Circular: Excellent structurally but not optimal for railway clearance envelopes in many cases.
• Parabolic: Rarely used as a complete tunnel section for railways.
• None of these: Incorrect because polycentric is a standard answer.

Common Pitfalls:

• Confusing TBM-bored circular bores with the final finished profile; fit-out can reshape the usable section.
• Ignoring aerodynamic and maintenance clearance allowances when selecting shapes.

Final Answer:

polycentric