What does a "D" plate on a signal mast indicate?

A "D" plate on a signal mast indicates that the signal is a Distant Signal. This type of signal is used to inform train crews about the status of signals ahead, allowing them to prepare for the conditions they will encounter as they approach the next signal. The purpose of a Distant Signal is to provide advanced notice about the upcoming block signal, indicating whether it will require a stop or allow the train to proceed at a certain speed. It serves as a warning and helps ensure safe and efficient train operations.

Understanding the role of various signal indications, such as Distant Signals, is crucial for train crews to maintain proper speed and make safe decisions while operating their trains. This knowledge directly contributes to overall safety and mitigates the risk of accidents on the railroad.