Railway wheel–rail interaction: What outward tread slope is provided on railway wheels to achieve self-centering and smooth curving?

Introduction / Context:
Wheel tread conicity (outward slope) is fundamental to vehicle dynamics. It enables self-centering on straight track and reduces flange contact on curves, improving ride, reducing wear, and limiting hunting at speed.

Given Data / Assumptions:

• Standard railway wheelsets on conventional track.
• Question asks the nominal outward slope used as a standard.

Concept / Approach:
With a coned tread, lateral shift changes rolling radii of the two wheels: the wheel with the larger effective radius runs on the outer rail, naturally steering the wheelset into the curve. The widely adopted nominal slope is 1 in 20, balancing curving ability (reduced flange contact) and high-speed stability.

Step-by-Step Solution:

Verification / Alternative check:
Vehicle/track dynamics references and permanent-way specifications list 1 in 20 as the nominal tread slope; variants exist for special cases but 1 in 20 is the textbook norm.

Why Other Options Are Wrong:

• 1 in 10, 1 in 15: Too steep; can increase sensitivity and hunting.
• 1 in 25, 1 in 30: Too shallow; poorer self-centering and more flange contact on curves.

Common Pitfalls:

• Confusing rail head cant with wheel tread slope; both may use 1 in 20 but are different design parameters.

Final Answer:
1 in 20