What signs are required to indicate the approach to the Working Limits?

The correct answer indicates that both an Approach Sign and a Stop Sign are necessary to effectively warn train crews about the imminent Working Limits. The Approach Sign serves as an advance warning, allowing crew members to anticipate the constraints of the Working Limits and prepare to reduce speed or stop as necessary.

The inclusion of the Stop Sign is crucial as it signifies that the train must come to a complete stop before entering the Working Limits, ensuring the safety of any ongoing work activities and personnel on the tracks. This dual-sign requirement aligns with safety protocols to minimize risks in areas where essential maintenance or construction is taking place.

The other options fall short in one way or another. For example, just a Passing Sign or a Stop Sign alone does not provide adequate warning in advance of the Working Limits as the combination of the Approach and Stop Signs does. Similarly, having only a Working Limits Speed Limit Sign does not convey the same level of urgency and awareness, as it lacks the critical element of alerting the crew to prepare for a stop and potential changes in their operational status.