Oldish post but i'll respond anyway.
The tunnels (Burt was talking about the railway tunnels) are still accessible (or at least were in August), though illegal and I was warned off by a hotelier and
told I would be arrested especially on a Sunday. Dont go through the tunnel from the station thats asking for trouble, drive up as far as you can and then walk up the
path that takes you to the green rail bridge. I have been through a few times and had no problems. But be warned.
Once through scramble down to the river and up the other side and get on the walkway, advisable to wear a harness and clip on, follow the walkway round, over the aquaduct and then abseil off on a pull through, 25m rope needed. There is at least one accessible hydro tunnel accessible from the start of the walkway.
Also I was informed by a guy guiding a couple there that there is another walkway about a mile and a half further on up the valley that has no protection. Somebody fell off and fell 80 feet surviving with a broken hip.
from the tunnel you can just make out the walkway
Here is a tunnel we did not explore as we had no light
The green bridge and the tunnels, the other end of the walkway is here and the abseil off, the walkway was chopped off here to deter tourists, I believe its possible to climb up at this point too.
The walkway is fairly safe but athere are a few exposed areas