In message
-sept
ember.org, at 11:46:23 on Thu, 24 Nov 2016, Recliner
remarked:

They might be to mark (and protect) the site in case the investigators
want to go back and look for more evidence buried in the ballast in the
near future.

Perhaps, though it would have made more sense to search the ground
thoroughly before laying the new ballast on it. Also, I don't think they
need to look for any more evidence for why and how the accident happened.

They won't know that in the general case, perhaps there's a broken bolt
they didn't find the first time.
--
Roland Perry