In reply to Tigger:
I was there just after Christmas. Definitive information was hard to find. We asked the man in the booth at the Chorro end of the walkway about access to Zeppelin and his reply was to walk up and over the ridge (though I suppose he can't say anything but that). We did this but it took so long that we decided there wasn't enough day left to try the route.
It was clear that plenty of climbers were walking through the railway tunnels. If I was there again I'd seriously consider getting up very early and doing this before anyone was around. It's equally clear that the fines are very high if you get caught. Personal choice I guess.
The tunnels are wide enough that you wouldn't have a problem if a train came, though it might be a bit noisy.
I can't comment on the state of the routes in the gorge I'm afraid, as we didn't actually get close to any of them.
If anyone has updates on this I'd be keen to hear them...