On the Midland Mainline, the next day I observed from the driving cab, "WE WILL BOMBE YOU" (with 'bomb' misspelled) which had been spray painted in white on one of the over bridges between Wellingborough and Bedford. I felt for the St Pancras drivers, quite a few of whom travelled into Liverpool Street from where they lived, before catching the tube around to Kings Cross St Pancras. They would've seen it heading into London on their last trip, having to get across London about an hour after seeing it. Whilst I don't believe it was a serious threat , it was a stupid, cruel thing to do.
The next day it had been heavily crossed out in white spray paint, either by the person who'd originally done it feeling remorse, or more likely Network Rail in a hasty attempt to get rid of it. It did look a slightly different paint, so I'd bet on the latter.
Bins on trains were locked closed for quite a while afterwards. Whilst this wouldn't protect against a suicide bomber, it did mean that trains were not delayed by something looking 'odd' in a bin, along with all the worry and fear that would go along with that.