We said 40 miles/h because the effects of derailment at that speed are far less devastating. No one can say there will never be another accident if trains travel at that speed. Travelling 40 miles/h will not necessarily prevent all accidents, but it will make people feel safer and cause less damage when events like those we've experienced occur, I am sure of this. If E=mc² still stands, if you double the speed, you quadruple the effects.
On April 18th, 2007. See this statement in context.