In permanent-way design, a switch (or ‘‘Smitch’’) diamond crossing is adopted when the diamond angle is very small. For what limiting crossing angle (in degrees) is a switch diamond typically provided?

Introduction / Context:
A diamond crossing is where two tracks intersect. When the crossing angle becomes very acute (small), ordinary fixed-rail diamonds suffer from impact, wear, and poor wheel guidance. Railways then adopt a switch (movable-point) diamond so flanges are guided positively. This question tests the threshold angle concept behind that choice.

Given Data / Assumptions:

• Diamond angle is the included angle between the gauge faces at the intersection.
• The term “Smitch” is a common misspelling of “switch” diamond.
• Practice refers to typical main-line standards used in Indian-type trackwork.

Concept / Approach:

For very small angles the gap at the nose becomes long, causing poor support and high impact. Movable switch noses remove the gap and guide wheels. The commonly cited practical limit for using a switch diamond is about 6 degrees or smaller.

Step-by-Step Solution:

Verification / Alternative check:

Track handbooks discuss changeover to switch diamonds in the 1 in 9–1 in 12 region (angles roughly ≤ 6–7°). Selecting 6° is a conservative and standard exam value.

Why Other Options Are Wrong:

8° and 10° are too large; 4° or 2° are possible but overly restrictive and not the typical exam threshold.

Common Pitfalls:

Confusing crossing number (1 in N) with angle in degrees; assuming fixed diamonds suffice for very acute angles.

Final Answer:

6°