I used to watch the iron ore trains pass the second steepest bank on their journey from Port Talbot to Llanwern near my home in Llanharan. Once the class 56's arrived in 1980 and the train length increased from 27 to 30 wagons, on occasions two 56's weren't available.
Margam Depot then had a problem so they put plan 'B' in operation and due to the fact 3 x 37's weren't allowed to haul 30 wagons, one 56 was used in tandem with 2 x 37's (So two crews were required). I saw this combination about 30 times, but have never seen pictures published reflecting this.
The empties would then return with the single 56 and 37's light engines prior. They would then either haul the next leg of the days roster or as was the case usually another 56 was available so the next service was 2 x 56's.
I attach a few images of some workings back in 1980/81 thanks Ian