When can rail traffic resume track speed after passing clearance boards?

The correct response is based on the understanding of the clearance boards' purpose in ensuring the safe operation of rail traffic. Clearance boards indicate the point at which a train can resume track speed, which is primarily concerned with the position of the entire train in relation to any potential obstructions or safety concerns.

When the rear of the train has passed the clearance board, it signals that the entire train has cleared the potentially hazardous area or the point of concern, allowing for a safe resumption of track speed. This emphasizes the importance of accounting for the full length of the train rather than just the front or other parts. It ensures that all rail traffic is completely clear of any restrictions or safety hazards before increasing speed.

The other choices revolve around the timing of when track speed can be resumed, which does not consider the implications of having the entire train clear of potential dangers. The focus on only the front of the train or the immediate passing of the board overlooks crucial safety protocols essential for maintaining the integrity of rail operations.