The paper provides an overview related to condition monitoring on railways, starting with a summary of key political and social trends for railways worldwide leading on to an identification of the parts of the railway for which efficient and effective monitoring is becoming an increasingly important issue. Railway condition monitoring has become a "hot topic" within the last 5 years in particular, although in the early days enormous amounts of data were collected without a proper understanding of how to extract useful knowledge from the data and how business processes needed to be adapted to take advantage of the monitoring information. Examples from within the railway industry of current good practice and appropriate application of advanced techniques are used to highlight developments that are currently being exploited. Some of the current research work on monitoring that has been undertaken by the authors and their colleagues will be used to illustrate the kind of advanced techniques that may become commonplace on railways in the future. Overall the paper reveals the principal trends for monitoring in railways, the associated research challenges and a future vision for monitoring systems and technology applied to railways.