He's been getting away with it for at least three years. London Underground
still haven't cleaned TOX.02 and TOX.01 off some trains and stations!

Are you sure its the same person? People who don't have the
imagination to do anything else than creep about painting the same
thing over and over and over again on railway property might not
possibly have the imagination to think of their own tag. The orginal
TOX might have grown up and working as an accountant in Green Park.
Or...
It could be a crew (as in gang not boat) who go by a group tag.