Just a thought.
The IRA bombed and murdered it's way around UK and Europe for around 30 years. Upwards of 3000 people died. Some weeks several attacks were reported of varying sizes and impact. Who remembers the lorry bomb in the city? The next day people were at their desks clearing out the glass and carrying on.
We've lived with 'not if but when' since 9/11.
It's taken Al Quaeda 4 years to strike.
Ok, it may just be the start of a campaign but I for one won't be scared off from using the tube.