Or alternatively a cryptic clue for Ha!! Wait Terror Hop. Hopefully someone wasn't trying to tell us something.
On that pleasant note if you want read The Survivalist's theories on how to survive a subway attack. The author thinks that underground riders are not prepared enough for what to do in emergencies.
"Although officials understandably would prefer riders to stay on trains and wait for emergency rescuers if at all possible, it also stands to reason that riders should have some sense of how to escape a train and navigate the tracks if absolutely necessary. What's the best way to break a window to get fresh air? What do underground emergency exits look like? How is it possible to determine if the electrified rails have been turned off?"
Fair points as it's all very well making sure that staff know what to do in emergencies but there's very, very little on London Underground trains about a passenger evacuation procedure - if such a thing exists. Perhaps TfL simply don't want to scare us into thinking about more attacks.
In which case I wish they wouldn't display cryptic messages like the above!