I think there were two Pontygwaith Halts (or platforms) one on the TVR and one on the line we are traversing.
Trevil Halt
FURTHER INFORMATION
If you look at the RAILSCOT line diagram on the Quakers Yard and Merthyr Joint Railway, there is a "closed stop symbol" with no station name but just a question mark that appears to be between the stations of Quakers Yard High Level and of Aberfan. Could that one be the Pontygwaith Halt on that line?