Originally Posted by
Lost_Luggage_in_SEA
My understanding (but I could be wrong and I defer to train/heathrow experts) is that the Piccadilly Line and Elizabeth Line run on different tracks at all times so that a problem with one shouldn't affect the other but this isn't true for HEX and the Elizabeth Line which do share the tracks for part of their route to Heathrow so it's possible for both to go down simultaneously.
You're right. The Elizabeth Line and Tube operate on entirely separate tracks, whereas the HEX and Elizabeth Line share tracks for almost the entire length of the route between Paddington and Heathrow.