Mystery solved

And it's not near as exciting as one may have hoped.

I went over to the yard this morning, hoping to talk to someone on the maintenance crew who could enlighten me as to the reason for exposing part of the roundhouse floor.
There I encountered a fellow, who's a yard sale buddy of my wife's, who works there and is a bit of a history buff. He told me that they had been given a front end loader to train on and he used that as an excuse to some minor amateur archaeology.
That's all.
Additionally they went into the "backside" of the yard and exposed some old pilings that supported a loading dock of some sort. I have to double check against the survey I have but I think it's an old stores building.
Not terribly exciting but certainly interesting to people like me.