S Stock is now replacing 50-year-old trains on the Metropolitan Line of the London Underground railway, with a commensurate step change in technology, functionality and performance. It is being introduced on to a railway that has its origins in Victorian times and includes the oldest section of underground railway in the world. This has presented a series of integration challenges that have had to be met. This presentation firstly covers the S Stock train itself, focusing on the features that are novel to London Underground and on the degree of system integration within the train that is necessary to meet passenger and operator expectations. The infrastructure changes required to support the new trains is then described, together with an overview of the system testing and validation work being carried out.