Sentence:  SWR <small>[South Western Railway]</small> said the move was a precautionary measure, understood to relate to electromagnetic emissions from the fleet causing changes of signal <WSD>aspect</WSD> in front of moving trains. 
Meaning of the word marked with <WSD> tags:  The visual indication of a colour light (or mechanical) signal as displayed to the driver. With colour light signals this would be red, yellow or green.