Track geometry check using three-axle level readings: Consecutive axles A, B, and C are 1.8 m apart with rail levels 100.505 m, 100.530 m, and 100.525 m respectively. What is the unevenness value (mid-chord deviation at B)?

Introduction / Context:
Rail unevenness over a short base is commonly assessed by comparing the middle level with the mean of the two end levels (three-point or mid-chord offset method). This measure indicates the local vertical deviation responsible for rough riding and dynamic wheel loads.

Given Data / Assumptions:

• Axle spacing (A–B and B–C) = 1.8 m each.
• Rail levels: A = 100.505 m, B = 100.530 m, C = 100.525 m.
• Unevenness at B is taken as the absolute mid-chord deviation: |B − (A + C)/2|.

Concept / Approach:
For three equal stations, the mid-chord deviation at the middle station is computed by subtracting the average of the two end levels from the middle level. This corresponds to the versine-like deviation used in track quality indices.

Step-by-Step Solution:

Verification / Alternative check:
The second-difference A − 2B + C = −0.030 m corroborates curvature; the corresponding mid-chord deviation equals 0.015 m (half the magnitude of the second difference for equal spacing).

Why Other Options Are Wrong:

• 0.035, 0.045, 0.055, 0.065 m: Do not match the mid-chord computation and would imply much larger local irregularity.

Common Pitfalls:

• Confusing total level differences with mid-chord deviation.
• Using maximum minus minimum level (0.025 m) instead of the defined unevenness at the center point.

Final Answer:
0.015 m