The maximum bending moment due to a train of wheel loads on a simply supported girder

1. Wheel Load: When a train of wheel loads moves over a simply supported girder, the bending moment changes depending on the position of the wheel loads.

2. Influence Line: The influence line for bending moment indicates that the maximum bending moment occurs when the wheel load is at a specific point on the span.

3. Critical Position: For the maximum bending moment to occur, the critical position of the wheel load is generally at a distance from the support, calculated based on the span and the load distribution.

4. Conclusion: Hence, the maximum bending moment due to a train of wheel loads on a simply supported girder always occurs under a wheel load.

Therefore, the correct answer is Option B: Always occurs under a wheel load.