Need to top and tail , as Merthyr and Aberdare are minimal single track terminii - Treherbet has sidings. The latter has passing loops to cntend with
Apart from major traction needs (yes - I suppose DVT's could be considered - but they waste platform space - as do engines ! ) - the track access costs would be higher as the tonnage passing over the lines would be significantly increased.
Not really an option IMHO.