They look like those auger (spiral) bits used for drilling holes prior to concrete piling, they connect together to extend the length of the auger as the hole gets deeper (or longer depending on direction) - so it's possible that both sections in the foreground came through the same hole.
So very serious as you rightly say, and if the auger sections had remained connected it could have spanned the tunnel and formed a pretty solid obstacle. (So far of course we won't be able to tell if the sections have been separated subsequent to the original incident.)
I wonder if RAIB have had to deal with this sort of thing before, thankfully it doesn't seem to happen regularly...