The original design was for lattice girders supported by brick piers resting on bedrock shown by trial borings to lie at no great depth under the river. At either end of the bridge the single rail track ran on top of the bridge girder, most of which therefore lay below the pier tops. However, in the centre section of the bridge (the “high girders”) the railway ran inside the bridge girder, which could then run above the pier tops to give the required clearance to allow passage of sailing ships upriver (e.g. to Perth).