These trains were what got me hooked to railways in the early 1980s. During 1981 they were worked by a pair of English 25s.
The trains from Leith carried light green concrete(?)-coated pipes for a gas field in Morecambe Bay (I think) though I don't know their exact destination. These were big diameter pipes, three or six per wagon? I think six, though I can't remember for sure, a long time ago!
The pipes were coated in Leith by a company called Bredero Price (at least that's who did them in the mid 1980s).
In 1981 the coated trains ran over the Edinburgh - Carstairs line and we saw them after school, so about 1600 through SW Edinburgh.
I remember seeing a pair of 25s on the train in November/ December 1981 but it was dusk so just too just dark to read the numbers. My dad took me to Millerhill the following weekend and the foreman thumbed through his book of workings to see if he could identify them. Alas not, because they had come from Leith and used the Portobello Jn - Niddrie Jn line to access the South Sub, thus avoiding Millerhill :-(
The use of 25s must have ended at about the beginning of 1982. My notes below show 47s working the trains in spring 1982. Pipe trains were still operating over the Carstairs line in 1983 but my records show pairs of Scottish 20s, and I'm not sure if they were to the same timetable or frequency.
And a bonus for this thread - some of the wagon numbers!
Sightings at Curriehill, Carstairs - Edinburgh line:
1982, undated but spring (May?) (still about 1600, IIRC)
47290 922546, 922826, 924502, 924601, 924583 (the trains were much longer, that's all I managed to write down) loaded pipes south
47013 924606, 924470, 922678, 923162, 922809, 922776, 924481, 924026, 923152, 924706 loaded pipes south
47321 924778, 922686, 922446, 922463, 922343, 923187 loaded pipes south
24 March 1983
12.34 20206, 20224 empty pipe train towards Edinburgh
922809, 923175, 922735, 922954, 924421, 922950, 922750
25 March 1983
12.53 20206, 20224 empty pipe train towards Edinburgh (so they had obviously taken a loaded train south in between time)
Also, one entry from Longniddry on ECML in May? 1982:
47051 pipe train towards Edinburgh 924731, 922523, 923158
Somewhere I have some of the Class 25 workings so I will post if I find them