Recently stumbled across a tunnel on the remains of the tramway between the colliery and the sand pits - which then I think went on to the iron works.
(relevant part starts at 6:12)
I'm guessing the sand pits just conveniently extended the line of the tramway to get their product to the mainline? I can't think of any reason a colliery would require large quantities of sand - the ironworks might have done for casting perhaps?