On the Bolton->Blackburn route, believe the highest point is in the vicinity of Sough Tunnel, about a mile from Entwistle station when heading towards Darwen, at 737 feet (225 metres) above sea level.
On the Burnley Manchester Road->Todmorden route, the highest point looks to be Copy Pit summit, about half a mile after passing through Holme Tunnel, and is some 749 feet (228 metres) above sea level.