When is the lamp held horizontally at arm's length used?

The lamp being held horizontally at arm's length serves as a signal for train crews, specifically indicating that they should reduce speed. This action alerts the crew that they need to prepare for a potential change in conditions ahead, such as a cautionary signal or an upcoming stop. Reducing speed allows for better control of the train and ensures that the crew can respond appropriately to any unexpected circumstances they might encounter.

The other options represent different actions that are communicated through distinct signals or lamp positions, reinforcing the importance of understanding specific signaling protocols in railway operations. Holding a lamp in other positions would convey different instructions critical to safe train operations, but a horizontal position specifically denotes the need to decrease speed.