Under which conditions does an engineer operate by absolute block?

The correct answer relates to the safety protocols for engineers operating trains, particularly in the context of managing speed and ensuring safety on the tracks. Operating by absolute block means that the engineer has explicit confirmation that no other trains can occupy a section of the track ahead.

When there is no automatic train stop in the head car, it is crucial for the engineer to maintain strict control over the operation of the train. In such cases, any potential risk of collision increases, making the absolute block system necessary to prevent accidents. This method ensures that the train can proceed only when it is confirmed that the entire block ahead is clear of other trains or obstacles.

The other conditions do not directly address the need for absolute block. For instance, the presence of a conductor or having all lights green might suggest operational readiness, but does not account for potential track occupancy. Poor weather conditions could affect visibility and safety, but they do not inherently necessitate an absolute block as a response to track occupancy. Thus, operating by absolute block in the absence of an automatic train stop serves as an essential precaution to guarantee safety in train operations.