What type of signal is described as never clear but always displays a red aspect?

The type of signal that is described as never clear but always displays a red aspect is indeed a marker signal. Marker signals are typically used to indicate the presence of certain conditions or areas that require attention, but they do not provide a clear directive regarding train movement, unlike other signal types. The consistent use of red in marker signals is a standardized practice to denote caution or the need for awareness, emphasizing that while the signal is evident, it does not give specific instructions about proceeding or stopping.

In the context of railway operations, this can help train operators to remain vigilant about possible hazards or changes in the environment without indicating a direct command to proceed or halt, which is reserved for other signal types. This understanding is crucial for ensuring safety and awareness in various operational contexts.