Under what circumstances may a movement continue if the engineer can no longer see a hand sign while pushing cars?

The correct answer states that the movement must be stopped unless a back up valve or back up hose is being used. This rule is in place to ensure the safety of operations when visibility is lost. The rationale behind this is that the engineer needs to maintain a clear and direct line of sight to any hand signals given, which are critical for safe operation during a push movement. If visibility is compromised, continuing to move without assurance of being able to respond to signals increases the risk of accidents or miscommunication.

Utilizing a back up valve or hose would provide an alternative means to control the movement safely, enabling the engineer to maintain oversight without needing direct line of sight to hand signals. This method effectively allows for continued movement while still adhering to safety protocols.

Understanding this context clarifies why stopping the movement under these circumstances is a necessary safety measure and why alternatives like a back up valve or hose are considered exceptions rather than routine practice.