During July 1838, William Chadwick, the contractor, acknowledged his responsibility for this occurrence. Remedial work was carried out before the centring was eased again in October 1838. The centring was then left in place over the winter. Author E.T. MacDermot has claimed that, as the bridge neared completion, the board of the Great Western Railway themselves had doubts that the arches would be able to stay up under the weight of passing trains and issued an order to Brunel, instructing him to leave the wooden formwork used to construct the arches in place. However, Brunel decided to lower this formwork slightly so that it provided no structural effect, but gave the appearance of being in place and supporting the bridge. Later on, the formwork was washed away during heavy flooding, but the bridge remained standing with no ill effects; in light of this happenstance, the strength of the arches was finally accepted and Brunel's design was vindicated. Operational life As built and opened on 1 July 1839, the bridge carried a pair of Brunel's "broad" gauge railway tracks over the Thames. Over the following decades, traffic to and from London increased enormously. During 1861, work was carried out to install mixed gauge tracks throughout the route between London and Bristol, thereby allowing standard gauge traffic to traverse the structure. In anticipation of the final conversion to the standard rail gauge, which was performed during 1890–1892, the bridge was widened on each side to carry four standard gauge tracks. This work was carried out under the supervision of the civil engineer Sir John Fowler, the width overall being increased from to . The expansion was undertaken sympathetically, resulting in the outward shape of the bridge remaining almost unaltered, but the new elevations and arches were constructed using redder Cattybrook brick. The prexisting London stock brick arches were also encased in Cattybrook brick to secure uniformity of colour, causing the distinctive chamfered step between the original Brunel arch and Fowler's additions. To avoid any differential settlement between the old and new sections, the foundation extensions were close piled and covered with a timber grillage, before being filled with concrete. During 1950, the western half of the bridge was awarded Grade II* listing; in April 1985, the eastern half also received the same level of listing. During July 2012, the Maidenhead Railway Bridge was upgraded to Grade I listed status by the Department