Cast iron arch of the original Barnes railway bridge, photographed on 7th July 2007. photograph by Gregory Beecroft

Cast iron is strong in compression, but not in tension. This had not been a problem with road bridges, because loadings were light. The weight and live loading of even the earliest trains imposed stresses that could cause fracturing and failure of cast iron components. This was demonstrated by the collapse of the Dee bridge at Chester in 1847. Railway companies strengthened bridges, usually with additional wrought iron beams, but continued to use cast iron arches.

Portland Road bridge, Norwood Junction, photographed on 11th August 2007. photograph by Gregory Beecroft

In May 1891 a cast iron bridge at Portland Road, Norwood Junction collapsed under a train. The Board of Trade Inspector who investigated the incident, Major General Hutchinson, found that there had been a flaw in a cast iron girder. However, even if the flaw had not been present, he considered that the bridge would have been under-strength for use by the heavier locomotives then working the line. He recommended the replacement of cast iron bridge girders by wrought iron or steel. The London, Brighton and South Coast Railway alone had to rebuild eighty bridges, and other companies replaced many others.

Most cast iron structures that were dismantled went for scrap, but Castle Street bridge, Salisbury escaped the blast furnace. This was a single span, with four parallel cast-iron arches. The cast-ironwork was reused in a new bridge nearby, carrying Nelson Street over the River Avon.

The arches were cast by Joseph Butler & Co at Stanningley Ironworks in 1857. The wrought iron lattice parapets and concrete piers were not part of the railway structure and date from erection of the bridge in its new location in 1898, photographed on 15th May 2010. photograph by Gregory Beecroft

Identical plaques attached to both parapets record the relocation of the bridge "without the aid of any machinery" under the direction of T. Scamell. The structure is now known as Scamell's Bridge. The layout of the plaque is curious, with the letters and words much more tightly packed in the upper half than the lower, photographed on 15th May 2010. photograph by Gregory Beecroft

Like many replacements for cast-iron structures, the present Castle Street bridge is of plate girder construction. The stone bands about three-quarters of the way up the abutments are the imposts off which the cast-iron arches were supported. The abutments had to be extended upwards to take the new plate girder span, photographed on 15th May 2010. photograph by Gregory Beecroft

Cast iron can still be seen quite widely in railway bridges, but normally in columns (where it is in compression, quite safely), lightly-loaded overbridges, or in components that are not load-bearing.