That was what I thought too, but watching the video there was quite a number which looked like a single pane of opaque or frosted glass as is found in the air-conditioned Mark 2def toilet windows, but they were always First Class and Mark 3 coaches (I'm not familiar enough with the differences to spot whether they were 3b on a moving target and wasn't looking out for the tail lights at the time).
see the example slightly to the right of centre
https://www.flickr.com/photos/44232489@N02/7752605128/in/photolist-cP58pE-bUsL2E-bUsKQs-pwXMTR-ca7meY-rey2yF-qSgeW1-A4DZ3T-UyrPLK-cbJD8f-ebWDJE-P1vTfX-95EunY-VdGEPt-bm4H3g-dGmCKL-norzhe-pZjsQT-pmKwiX-qqR8U9-bUsLYJ-9xdYLP-atougW-ca7nmd-9vLTfc-atkQFn-bUsKLW-gJSsCJ-pofuKZ-dwyHEC-cXmvG9-qUtKRG-axigww-m1dHVo-rpsz2r-nuuHGm-dCwJAU-gMeKjU-bxQbEx-T9eT8c-ojLkF3-U9Wjny-bGThwa-qTWwx6-eS36SJ-qD9Zjm-Tbxi7v-rhorfz-dBeHd9-nhpLmi