50. Network Rail shall not in the course of constructing any specified work or protective work or otherwise in connection therewith do or permit anything which may result in the pollution of a waterway or the deposit of materials therein and shall take such steps as the engineer may reasonably require to avoid or make good any breach of its obligations under this paragraph; but this provision shall not prevent Network Rail from carrying out works to the abutment of bridges or other works within the waterway.