It is part of the Dudding Hill Line, which is a freight line running from Acton to Cricklewood and was once a passenger line https://en.wikipedia.org/wiki/Dudding_Hill_line https://www.openstreetmap.org/#map=15/51.5309/-0.2545&layers=T
Guessing they scraped the route from google maps or something to develop the line but failed to erase that part
I can’t remember correctly, but I thought they said that they removed from the map all of the WCML track so that people didn’t get confused. Maybe this was just a part that they missed out.