The Ferry Road subway could still be there, albeit flooded - but the location duplicates Pont y Werin. Most of the Glamorganshire Canal "hole" under Queen Street was filled in but the setion under Kingsway is in use as a subway. The Stadium House cable tunnel is still in use by BT. I believe it runs from Castle Street to Fitzhamon Embankment. There are none others that I can think of in the city centre.
I think the tunnels from Cardiff Castle to various places (everywhere from Castell Coch to City Hall) belong in the 'urban myth' category.
The "urban exploration" forum's section on Cardiff has some good reads on the city's hidden places.
Somebody once told me there is a tunnel that runs (used to run?) from near the Heath Hospital to somewhere near the city centre - and apparently the nub of the entrance near the Heath is still visible just off a footpath by the Gabalfa Flyover - I'd link you on Google maps but I can't quite find it myself (vague I know!).
There's definitely a tunnel running under Castle street!
yes ther are tunnells under cardif they belong to bt 1 runs from maryann st to fitshamon embankment fromthere to the castle castle to wesgate st the bt building contains 2shaftsits not big enough for a train thogh lol i no i worked in them under compressedair its another world down there