Noel wrote:...
Another possible answer has occurred to me - could the train could be an up train, but standing "wrong line" to let another one past? The visible 'up' signal is off, which might support this.
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Possible, but unlikely I think as the train is loaded, with wagons from companies working further up the valley, so almost certainly a down train. There's a brake van in the appropriate place too for a down train.
I'd wondered if the loco were there to help reverse the train into a layby siding, but there isn't one close by. The thought crossed my mind, when looking at the map of the area, was that the loco could facilitate picking up wagons at Trosnant Junction coming off the "third line" of the Taff Vale Extension line.
A couple of extra observations.
A nice mixture of wagons with differing buffer arrangements, and variations of wagon livery with lettering of Blaenavon wagons going on to the corner plates.
If the loco is really at the end of the train, what would the tail lamp arrangements be? A brake van at the end of a train would have normally had side lights: would these be removed if a banking engine were coupled to the train? In this particular photo the resolution is not good enough to be certain whether there are, or are not, side lights on the van.