An interesting post,Darren.I was born and raised in the Aberdare valley and can remember the last years of train services....passenger,coal and other freight...until their eventual demise in the early to mid 1960's.
FYI,there were two lines.The first,which made connection in Aberdare with the GWR Vale of Neath line,was the ex-TVR branch which ran from Aberdare Low Level to Abercynon,where,on most occasions,you changed to the Merthyr to Barry Island service which took you via Pontypridd and Taff'sWell to Cardiff Queen Street or General.Most passengers from the valleys gravitated to Cardiff.
The Aberdare branch was ,until 1958 and the arrival of the DMU,operated by ...from 1953 to 58....an Auto Train....64XX powered.This,at peak times,consisted of trailers at both front and rear,with the loco and a Hawksworth suburban sandwiched in the middle.
On arrival at Abercynon,via Mountain Ash (there were two separate stations there as well) ,passengers alighted and the auto train quickly reversed back up the Aberdare branch to a siding. This was because Abercynon was an island platform and it had to be kept clear for the upcoming service to Merthyr from Barry Island.We Cardiff passengers crossed over to the other side to await the service coming down the bank from Quaker's Yard and Merthyr.When both up and down services had cleared,the Aberdare Auto Train returned to the platform to collect passengers from Cardiff and made its stately way back up the branch to Aberdare.So hurry up,Bachmann with the Auto Train!
The other line was,of course,the ex GW Vale of Neath,which carried freight of all kinds,powered by panniers ,28XX,42/52/72XX,56XX,Halls,Granges,Manors,ROD's,the occasional WD.The passenger service from Neath to Pontypool Road was usually a 2 to 4 coach load.This would have anything at its head.....panniers and 56XX,41/51/81XX 2-6-2 tanks,53XX 2-6-0.In the mid 50's ,Manors 7828/29 were allocated for a short while to Pontypool Road for the service.The odd Grange also put in an appearance.
Sadly.....no Dukedog,I'm afraid. Never mind it is your layout and it is perfectly feasible to say they COULD have been used.
Don't forget NCB saddle tanks....there was one called 'Lord Camrose' at Mountain Ash....