I would rather see it used as a rail tunnel again. There have been some lines re-opened in the general area, and extending the line from Treherbert through the tunnel to serve numerous communities which lie beyond would seem to be a good idea. The big problem with communities which are in almost parallel valleys is that they are somewhat isolated from each other and a bit of public transport linking them ought to be beneficial socially and economically.
Had it been double track, there would have been the possibility of putting a barrier in the middle and running trains alongside cycles and even pedestrians. But no-one will be boring it out to double size.
There is at least one long tunnel in Alaska which is shared with trains and road vehicles. Not sure that it would be possible with cycles. How do you prevent someone getting his wheels stuck in the flange gap? I have vague recollection of a recent invention which does that, basically a rubber filler which compresses under the weight of a train but not anything less, but I can't find any details. I do remember the Connel Bridge in Scotland being successfully shared between road and rail.
https://en.wikipedia.org/wiki/Portage_Glacier_Highway