I was sitting on a stile by the Wirksworth Branch one day and this coach came ghosting down all by itself. Just a hole cut in the end for the driver's windscreen as I remember it (long time ago) It was the test bed for the mechanism that was to be fitted to the
Derby Lightweights, most of which were also tested on the branch.
In general they were two and four car sets, but one of the two single units returned to the BR Research, when I was there, and christened "Iris", and is now in retirement on the branch, operated by the Ecclesbourne Valley Railway. So she and I are old friends.