Why do some dead end sidings have a small white light above the red light?

The presence of a small white light above the red light on certain dead-end sidings serves an important signaling function. Specifically, it is designed to avoid any potential confusion with the lights used for running signals.

In a railway context, signals are critical for ensuring the safe movement of trains. A red light typically indicates a stop condition, and in the case of dead-end sidings, it informs train operators that the track does not continue and that they need to halt their movement. The addition of a small white light provides clarity, signalling that this particular stop is not a running signal but rather an indication of a dead-end siding. This differentiation is crucial for operational safety, as it helps train crews properly interpret the signals in different contexts, thereby reducing the risk of errors and ensuring that safe procedures are followed when approaching these areas.

In this context, the other options do not adequately describe the main function of the white light.