What happens or what is the procedure? When they come across something
such as a plague pit when digging Crossrail or a tube line?

I'm not a virologist or a biologist, but is there not a worry/concern of
releasing the virus if these sites are disturbed? I know that we can
treat something like bubonic plague at this time, but I also understand
that treatment is a long and difficult task as bubonic plague remains a
nasty piece of work and can do quite a bit of damage in any event.

Besides concerns of plague, I wonder if diggers and builders have ever
come across something that scared the hell out of them.

Alternatively, has a route trajectory drastically changed with no
explanation?