Before 54, the MTA line had Stanier 3 2-6-2 tanks and Ivatt2-6-2 tanks. On the odd occasion, one would get exLNWR 0-8-0s and Coal tanks.
The Hirwaun line had 64xx, very early on a 14xx was used. The miners’ trains were hauled by 56xx
You never got outside cylinder ex-GWR locos, as they couldn’t get through Troedyrhiw and Merthyr Vale stations, so no 51xx.
The Pontsticill service was mainly 64xx with auto coaches, but 57 and 97xx were used as well, still with auto coaches but running around at both ends of the line.
There was often (always) a station pilot in Merthyr before 58 as Merthyr had separate incoming and outgoing platforms for the non-autocoach services.
The go-to locos in Merthyr were panniers, and they operated most of the freight services; double-headed on Brecon freights.
The odd ex-TV loco came to Merthyr as well.
I wish I had been a spotter then!, Dai