Might be indulging in a little bit of optimism but could coryton be in preparation for Coryton extension and tram usage? And Ystrad be for a future reopening (to passengers) of a line to Nelson and Trelewis using trams? Hence no need for Rhymney?
Back to reality it could be gradients to Rhymney being a blocker but I thought tram trains could handle steep gradients?
Back to reality it could be gradients to Rhymney being a blocker but I thought tram trains could handle steep gradients?