Latterly, I believe it was mainly Stanier 8F 2-8-0s on the end-to-end trains (Shrewsbury to Swansea Victoria - no reversal at Llanelli in those days) and ex-LMS 2-6-4 tanks on the local short workings at either end of the line. This ex-LNWR line was one of the few routes on which you could enjoy 8F haulage on a regular basis, as they were preferred to Black 5s because their smaller driving wheels and higher tractive effort gave better acceleration on the steep gradients, where a high maximum speed was not a priority.
There is an interesting thread about this line, on "Railway History and Nostalgia". Unfortunately my computer skills are poor, and tend not to extend to linking to threads -- at all events, thread is titled Central / Heart of Wales Line -- historical. Its commencing date is 28 Dec. 2018 -- currently on that sub-forum's "page 82". (Would be extremely grateful, if some kind person were to do the necessary to link to the thread.)
OP of thread includes a video -- poster remarks, "rather poor quality, but very interesting film" -- not quite 10 minutes long, shot in 1961: basically following in short clips, a northbound passenger train along the route -- many locations fleetingly shown, including other train / motive power action encountered during the journey -- shots taken between Swansea (Victoria), and Craven Arms. The motive power of the train featured and travelled on, is (unusually for the line, one gathers) a Jubilee 4-6-0.
The thread's subsequent posts include a couple of comments on the present-day "tree ubiquity" along the route: its tendency to spoil views, and the desirability of removal of at least some of this "cover" !