The bridge contractor was William Evan. The ironwork was constructed by Easton & Amos. On 15 June 1846, the foundation stone of the towers was laid. The towers accommodated entrance portals within twin arches, through which the railway are carried into the tubes. The tubes are made of 16mm riveted wrought iron with cellular roofs and bases, and sheeted sides; each one weighs roughly 1,320 tonnes. The tubes, which are 129.2 meters long and 4.4 meters wide, were constructed using shipbuilding techniques. Once completed onshore, they were attached to pontoons, floated along the river and jacked into position between the abutments; steam-powered hydraulic engines lifted the bridge elements into place. On 6 March 1848, the first tube was floated; its installation took nine days to complete.