There was a short segment in the above programme on the work to lower the track to allow for electrification. I think it was towards the end of the programme - in fact I believe it was the last topic - just after a man dressed as a bird table and an attempt to get a stag to rut with a fake deer. This was on the programme on 17/11/2015
There were a couple of sequences of
HSTs▸ entering the tunnel but me thinks -given the livery of one set was similar to the one shown below - one of the clips may have been quite old
http://www.penmorfa.com/Archive/eighteen.htmNote this link is only intended to show the livery and is nothing to do with Box Tunnel!
UPDATE
It's on iPlayer at the link below at about 24:00 in
http://www.bbc.co.uk/iplayer/episode/b06p3r67/the-one-show-17112015