What and where: the answer

Yes, as Adam and Steve have identified, the photo is of one of the many square holes in the "new" concrete-lined section of Blisworth Tunnel.

These allow original openings in the tunnel to discharge water and are fascinating to look at with a torch as you pass. My camera's flash isn't good enough to illuminate the interesting passageways, but below I've tweaked another photo, this one of the side shaft perhaps leading to an older alignment of the tunnel. Adam has written about this and taken a photo of it here.