Scale of attack, amount of force required, social and economic impact of the accident.
Look over a list of who the victims of the Titanic disaster were -- comparable in that it was the luxury travel alternative of its day. Jay Jacob Astor, Benjamin Guggenheim, Ida Straus (founder of Macy's), just to name a very few. The societal impact of the Titanic's loss was ... titanic.
You wouldn't even need an explosive to foil the tube. Simply something to perturb the system slightly would disrupt it in a bad way. Even if only a single small transport were in the vicinity, any others in the tube would also be doomed (granted, at 1 hour/trip, headways would likely limit this to a very few instances).
Trains are very robust. Conventional high-speed rail is a bit riskier, but even accidents such as on the German Eschede rail disaster (at 200kph) saw over 60% of the passengers survive: https://en.wikipedia.org/wiki/Eschede_train_disaster
Look over a list of who the victims of the Titanic disaster were -- comparable in that it was the luxury travel alternative of its day. Jay Jacob Astor, Benjamin Guggenheim, Ida Straus (founder of Macy's), just to name a very few. The societal impact of the Titanic's loss was ... titanic.
You wouldn't even need an explosive to foil the tube. Simply something to perturb the system slightly would disrupt it in a bad way. Even if only a single small transport were in the vicinity, any others in the tube would also be doomed (granted, at 1 hour/trip, headways would likely limit this to a very few instances).
Trains are very robust. Conventional high-speed rail is a bit riskier, but even accidents such as on the German Eschede rail disaster (at 200kph) saw over 60% of the passengers survive: https://en.wikipedia.org/wiki/Eschede_train_disaster