12 July 1841. Initially, there was only one track across the structure in operation; the second line, along with the viaduct's ornate stone parapets and pavilions, was not completed until the following year. By 1846, the viaduct had become part of the London, Brighton and South Coast Railway. In 1923, as a result of the Railways Act 1921, it became part of the Southern Railway network. It remained under Southern's ownership until January 1948, when the nationalisation of the Big Four railway companies formed the publicly owned railway operator British Railways. Maintenance and restoration The first major restoration work occurred during the 1890s, and was focused on repairs to the brickwork. Engineers of the late Victorian era were concerned that the original lime mortar used in the viaduct's construction was inadequate and the decision was made to replace it with cement mortar. However, the replacement facing brickwork and substandard mortar eventually caused its own failures prompting more expensive repairs later on. This was likely due to the repair work having borne a greater share of the structure's load than intended, resulting in an accelerated failure rate. Poorly bonded header bricks are another probable culprit for its ineffectiveness. Additionally, the parapets and pavilions, although made from Caen stone (a high-quality limestone), have been subjected to heavy weathering. By 1956, the damage to the viaduct was extensive but the cost of refurbishment work was deemed too high by British Rail. The degradation was partially a consequence of the structure's long lifespan: when originally constructed in the 1840s, its intended design life was only 120 years. By May 1983, the viaduct had been officially recognised as a Grade II* listed structure. By the 1980s, the eight pavilions present on the viaduct were in such a poor condition that some of their roofs had fallen in and the installation of internal props was required to halt their further collapse. As a consequence of its heritage status, all envisioned alterations to the viaduct need to be reviewed and agreed upon by English Heritage. When British Rail proposed to dismantle the original pavilions and rebuild them using reconstituted stonework, English Heritage refused permissions; accordingly, there was no substantial restoration performed to the viaduct during this period, a decision which British Rail publicly attributed to the sizeable estimated cost of such works. The fabric of the structure continued to deteriorate over the next decade, with sections of