Permanent way – switch selection: For achieving the best operational performance and reduced maintenance at turnouts, which type of switch is generally preferred on modern railway track?

Introduction / Context:
Turnouts are critical assemblies in railway permanent way. The design of the switch (tongue rail against stock rail) strongly influences ride quality, impact forces, wear, and maintenance needs. Among historical forms—straight-cut, undercut, and overriding—modern practice standardizes the geometry to minimize wheel impacts and provide smooth entry to the diverging route.

Given Data / Assumptions:

• Mainline turnouts on standard modern track.
• Objective is best performance: smooth guidance, reduced impact/wear, better service life.
• Correct chairing, fastening, and rail profile assumed.

Concept / Approach:
An undercut switch is planed on the underside of the tongue head/web, allowing the top running surface of the tongue to remain flush with the stock rail running face when closed. This creates a near-continuous running surface, reduces the wheel flange contact shock, and improves both safety and service life. Straight-cut and especially overriding forms lead to higher impact and wear at the switch toe.

Step-by-Step Solution:
1) Performance goal: minimize wheel-rail impact at the switch interface.2) Compare switch types: straight-cut vs undercut vs overriding.3) Undercut retains flush top running, reducing impact and promoting smooth guidance.4) Conclude undercut switch is preferred for best overall performance.

Verification / Alternative check:
Track standards and maintenance reports consistently show reduced battering at the tongue toe, lower wear rates, and improved geometry retention with undercut switches compared with older straight-cut or overriding designs.

Why Other Options Are Wrong:

• Straight-cut switch: simpler but causes more impact at the toe.
• Overriding switch: increased risk of topping and surface mismatch.
• Both (a) and (b) / None: only the undercut form aligns with the stated performance objective.

Common Pitfalls:

• Confusing switch type (undercut) with switch plan geometry (straight vs curved switch—another concept).
• Ignoring correct planing and maintenance, which are essential even for undercut switches.

Final Answer:
Undercut switch.