I know very little about this, I only picked up on its existence from a Flickr photo. It follows the path which exits the wood so its highly likely its to do with the ironstone mines in Eston.
The track continued in a cutting on the other side of the road, heading off towards Dunsdale, although on closer inspection that is to the north of this area.
A tunnel is marked on this map at Waggonways which also suggest a second tunnel closer to Dunsdale
By way of a disclaimer and warning, its located on a scrambler track (possibly on private land?)
It’s likely unsafe, you’ll get very dirty and need a torch.
*Update 2008* Having visited more mining sites subsequently and spoken to other, I now believe this to be actual ironstone workings rather than a rail tunnel. Its unlined and a bord and pillar arrangement of passages is running parallel to the road not just under it, here are some internal shot taken with a better camera.