The route north of Coryton would follow the old alignment, as far as Longwood Drive. It would then depend on exactly where the J4 P&R would be sited - I assume the parking would be inside the roundabout but it is not neccesary to place station inside - it could be just outside - connected by pedestrian underpass.
The route would then run parallel to south of M4 before crossing river
And yes - the plan is to use tram-trains
Purely for speculation purposes, I have been looking at how to 4-track the section of lines to the north of Queen Street Station up to the Rhymney-Taff lines junction.
I would suggest adding one track either side of existing Newport Road Bridge. There is already an unused abuttment to the west which could be utilised. A new abuttment would need to be constructed to the east side.
The new track would conflict with the existing car park to the west - but this could be resolved by a very limited demolition at the south east corner of the building. The extent would depend on the structural arrangement of the car park, but it appears to be feasible, with out much loss of parking capacity.
A new track on the east side would encroach on the university buildings but would probably not conflict. A RC retaining wall would probably be required here.
Alternatively, place two new tracks to west of existing bridge, making full use of existing abuttments. This would avoid encroaching on university buildings, but would require more extensive demolition and remodelling of the car park.
Both options appear to be feasible.
If this could be done, would it be better to use Queen Street plaforms 1&2 for Taff Line services, with Rhymney Line using platforms 3 & 4, or better to use 1&2 for up lines and 3&4 for down lines ?.
