Investment in urban rail systems has increased rapidly over the past decades. While there have been some successful systems, a significant number have failed to be as successful as expected in improving public transport and urban environment. These experiences have led to a considerable debate in the literature about the effectiveness of new urban rail systems and whether it is worthwhile investing in them. While these debates continue, political support for metros, and particularly light rail systems, is still strong, and investment in them is very likely to continue. Hence, it is extremely important to understand the factors that make these systems successful. The paper presents the findings of a research aimed at identifying these factors based on the analysis of eight new urban rail systems: four in the USA, three in the UK and one in Canada. The paper presents the results of the performance analysis of the case studies as well as a detailed analysis of the factors that affected the performance of each system. Based on the case study analysis, a comprehensive list of factors that make urban rail systems successful is presented. The analysis also facilitates a comparison between the experiences of the three countries, and focuses on the effects of different operating environments and different planning approaches on the performance of urban rail systems. The paper concludes with a comparison of the findings of this research to those of previous studies.