What aspect of rail signal systems does "rare" imply?

The term "rare" in the context of rail signal systems signifies that the signals are infrequently employed or utilized. This indicates that their application is not common, and they may be used in specific situations or under particular conditions rather than being part of the regular signaling protocol. This contrasts with the notion of signals that are typically used, which are part of everyday operations.

In this context, standardized signals would imply a universally accepted set of signals that are commonly utilized across various railroads, while signals for freight only would suggest a category strictly reserved for freight train operations, neither of which aligns with the idea of rarity. The focus on infrequent employment is crucial as it highlights the specialized nature of the signals referred to by the term "rare."