One such place that you may have seen is the almost-completely-subsided "Indiana Jones" track you can sometimes make out on the southbound line from Treforest to the Estate, Is it still there? And when was it severed from the main line?
Maybe something to do with
this?
There's loads of rail archaeology all the way up the Taff Valley. Mostly abandoned lines caused by the obstructive attitude of the Taff Vale Railway Company, whose route is the present line (designed by Brunel incidentally).
There's
this as well a little further down the valley near Taff's Well.
Anorak with fur-lined hood, methinks.