Track geometry term: The “gauge” of a permanent way is defined as which distance relative to the rails?

Introduction / Context:
Gauge is a fundamental dimension of track; it defines compatibility with rolling stock and directly affects stability and capacity. The precise definition ensures consistent wheel-rail guidance across the network.

Given Data / Assumptions:

• Two parallel rails forming a track.
• Running faces are the inner faces contacted by wheel flanges.
• Measurement taken at a specified distance below the rail top per standards.

Concept / Approach:
Gauge is specified as the minimum distance between the running faces of the two rails. Measuring between running faces captures the actual guidance clearance for wheel flanges, unlike centre-to-centre or outer-face dimensions which vary with rail section and wear.

Step-by-Step Solution:
1) Identify the functional faces guiding the wheels: the running (inner) faces.2) Define gauge as the minimum spacing between these faces.3) Ensure measurements follow standard height from rail top to avoid head profile effects.

Verification / Alternative check:
All major standards (broad/standard/metre gauge) define gauge using running faces, confirming the correct option.

Why Other Options Are Wrong:

• Outer faces or centre lines do not dictate flange guidance.
• Formation width is unrelated to rail gauge.

Common Pitfalls:

• Measuring at the rail head top corner instead of prescribed datum below top where profile is stable.

Final Answer:
Minimum distance between running faces of the two rails.