What is the required action when a train with flat cars has stopped and the motorman wishes to proceed?

When a train with flat cars has stopped and the motorman wishes to proceed, sounding two blasts of the horn is a standardized signal to communicate intentions to others, particularly those operating in the vicinity. This action serves as a clear indication that the train is about to move, providing essential information to anyone who may be in the area, including crew members, flagmen, or other trains.

Sounding two blasts is a general practice that enhances safety by alerting others to prepare for movement. This is especially important with flat cars, as they may not have the same visibility or presence as other types of cargo or passenger cars, and ensuring that all personnel are aware of the train's intentions is crucial to maintaining a safe operating environment.

The other options, while they may seem relevant under specific circumstances, do not align with the typical protocols for indicating a train's intention to move. Signal the flagman might be necessary if there were other factors at play, such as signaling to a crew member further along the track. Sounding one blast of the horn generally indicates a different action, such as warning or signaling for acknowledgment, and checking for clearance is an important step but not represented by the primary action of announcing the intent to proceed.