Wheelset geometry: The distance between the inner faces of a railway wheelset’s flanges relative to the track gauge is kept…

Introduction / Context:
Safe and smooth wheel–rail interaction depends on precise clearances. The “back-to-back” distance between the inner faces of a wheelset’s flanges must accommodate the running gauge and provide clearance for rail head and check rails. This question targets the standard relationship between that flange spacing and the nominal gauge.

Given Data / Assumptions:

• Conventional railway track with fixed gauge.
• Wheelsets with coned treads and flanges.
• Need for operational clearance to avoid flange binding.

Concept / Approach:
The inner faces of flanges cannot be exactly equal to the gauge—some clearance is essential to prevent tight running, allow curving (angle of attack), and accommodate manufacturing tolerances and thermal/elastic movements. Therefore, the flange inner face spacing is kept slightly less than the track gauge.

Step-by-Step Solution:

Verification / Alternative check:
Railway standards specify a defined back-to-back tolerance that is less than gauge to ensure safe passage through curves and switches without flange climb.

Why Other Options Are Wrong:

• Equal to gauge: Leaves no clearance; risks binding and accelerated wear.
• Slightly more than gauge: Excessive clearance; poor guidance and potential hunting.
• None of these / midway statement: Not relevant to flange spacing specification.

Common Pitfalls:

• Confusing gauge (rail spacing) with back-to-back (wheel flange spacing).
• Ignoring tolerances essential for curving and thermal effects.

Final Answer:
slightly less than the gauge distance