What is the hand signal to indicate Reduce Speed?

The hand signal to indicate "Reduce Speed" is the one that is held horizontally at arm's length. This signal effectively communicates to train operators or crew members that they need to decrease their speed, providing a clear and unambiguous message.

Holding the arm horizontally at arm's length is a widely recognized and standardized gesture within rail operations, making it easy for personnel to recognize from a distance. This clarity in communication is crucial for maintaining safety on the tracks, as it allows for proper responsiveness to the directive given.

The other options represent different signals that are used for various other commands or indications, which may not convey the intention to reduce speed clearly. Thus, using the recognized horizontal arm position helps to avoid any confusion that could arise from misinterpreting or misapplying different signals in the context of rail operations.