If additional instructions are not received during shoving movements, what must the crew do?

The appropriate action when additional instructions are not received during shoving movements is to ensure that the movement must stop within half the distance specified. This is a safety protocol designed to prevent accidents and ensure the crew has adequate control over the situation. Shoving movements can be inherently dangerous due to the limited visibility and potential for unforeseen obstacles or conditions. By requiring the crew to stop within a specified distance, it provides a buffer to protect people and equipment. This practice underscores the emphasis on cautious operations in the rail industry, prioritizing safety over speed or efficiency.

Considering the other options, validating verbally or checking signals may not necessarily address the scenario of not receiving additional instructions during a shoving movement. Increasing speed for efficiency directly conflicts with safety protocols and could lead to unsafe conditions, which is why those alternatives are not appropriate in this context.