Which of the following describes the recommended speed for running on straight track in a yard?

The recommended speed for running on straight track in a yard is referred to as "Series speed." This term is used to indicate a specific operational speed that is safe and efficient for train movements in a yard setting. It encompasses the guidelines for how trains should be operated in yard locations where conditions can vary significantly due to train handling, the presence of personnel, and the proximity of track infrastructure.

While there are specific numerical speed limits for running trains (like 10 MPH, 15 MPH, or 25 MPH), the term "Series speed" reflects a broader operational practice that is based on factors such as train type, yard activity, and safety protocols. The exact numerical speeds can vary by railroad, but the concept of Series speed captures the essence of safe operational practices tailored for the diverse conditions and activities occurring within railroad yards. Thus, identifying it as Series speed encompasses the principles of safety and operational efficiency in yard environments.