Maintenance of points & crossings: A wing rail shall be renewed or reconditioned when its maximum vertical wear reaches (use standard maintenance units).

Introduction / Context:
Wing rails are vital components of crossings (frogs) guiding wheel flanges through the gap near the nose of crossing. Excessive wear jeopardizes guidance and safety, so clear limits trigger renewal or reclamation.

Given Data / Assumptions:

• Vertical wear measured at specified reference points on the wing rail running surface.
• Standard maintenance thresholds are used in millimetres.

Concept / Approach:
Wear limits ensure the rail still maintains the correct geometry to support and guide wheels without flange climb or impact damage. For wing rails, a typical limit is around 7.5 mm of vertical wear before action is required.

Step-by-Step Solution:
Monitor vertical wear periodically using templates/gauges.Compare measured wear to intervention threshold.At 7.5 mm vertical wear, schedule renewal or reconditioning.

Verification / Alternative check:
Crossing maintenance manuals prescribe lower limits for wing rails than for some plain rails due to geometry sensitivity.

Why Other Options Are Wrong:

• 5.0 mm and 5.5 mm: Conservative but below common practice; may lead to premature renewal.
• 9.5 mm: High for wing rails, risking poor guidance and impact.
• 12.0 mm: Excessively high; unsafe before reaching this wear.

Common Pitfalls:
Confusing limits for nose of crossing, check rails, and wing rails; reading wear at wrong location.

Final Answer:
7.5 mm