Sentence:  These old <WSD>indicators</WSD> <small>[at Earl's Court station]</small> show the destination of the next train by the appearance of an illuminated arrow next to the station name. The arrow may indicate that a train is going to Wimbledon, but it gives no clue as to "when". Now, however, the <WSD>indicators</WSD> are supplemented by dot matrix panels that not only say where but also when. Why aren't the old ones removed? Because they, like the whole station, are Grade II listed. 
Meaning of the word marked with <WSD> tags:  A pointer or index that indicates something.