that used to cross the valley near the pit between Woods glass works and Beatson Clarks (the winding gear is still there as a "monument") The bridge was a very high girder type structure, if I remember right, and stretched across towards Cundy Cross, over what is now the Dearne Valley park. Pics would be good.
You just have to look up Cudworth Flyer online and u will see steam trains going over the viaduct. It was there in the 1960s when I used to play round there
Called the Oaks Viaduct in fact http://upload.wikimedia.org/wikipedia/commons/thumb/d/d6/Oaksviaduct.jpg/330px-Oaksviaduct.jpg