Well, I'll see what I can do re pics:
Essentially barrier coaches are a basic mk2d with the windows welded
over, and in whatever livery !
Test train stock is identical, usually in a grimy off-white colour.
Usually 1 coach bears "area" (ie Doncaster Test Train) they are
usually in a rake of 5-7 coaches. Commonly used for "shaking down"
re-fitted locos for example the class 91's or training drivers on 47's for
T'bird duties.
Example of a barrier coach at Craigentinny: Mk2d, with windows welded,
Painted in an all over GNER blue, with red stripe but no "Route of F.S."
Logo. Bearing the legend in white "IC125 Barrier Coach" halfway between banding and coach bottom.
There are numerous Mk 1's with welded windows - purpose debatable!
I don't have any pics of these coaches - but the dimensions are obviously
"per coach" - if you got hold of a pic of a test train then all these coaches are basically the same.
An excellent site I have found:
http://www.samsonrail.com (Steve Jones' Rail pics page) Well worth a look - some VERY grimy BR stock!