Philip
On Moderation
You need to start off with railway tunnels that it's perfectly legal to enter, for example those on the Monsal Trail in the Peak District.
Swainsley Tunnel (on the old Manifold Valley Light Railway) is another I find pretty atmospheric. It's straight so it's not as though it's going to get too dark - but it's narrow-gauge so it's smaller than you'd expect. It's now part of a public road so it's still very much in use!
Harecastle south sounds like a real daredevil and thriller job, but does this still come under 'legal'? It has gates over the portals but this doesn't mean it is trespassing to go in, could be because it is deemed risky.