Urban rail systems are facing increasing pressure to minimise their energy consumption and thusly reduce their operational costs and environmental impact. However, given the complexity of such systems, this can only be effectively achieved through a holistic approach which considers the numerous interdependences between subsystems (i.e. vehicles, operations and infrastructure). Such an approach requires a comprehensive set of energy consumption-related Key Performance Indicators (KEPIs) that enable: a multilevel analysis of the actual energy performance of the system; an assessment of potential energy saving strategies; and the monitoring of the results of implemented measures. This paper proposes an original, complete list of KEPIs developed through a scientific approach validated by different stakeholders. It consists of a hierarchical list of 22 indicators divided into two levels: 10 key performance indicators, to ascertain the performance of the whole system and complete subsystems; and 12 performance indicators, to evaluate the performance of single units within subsystems, for example, a single rail vehicle or station. Additionally, the paper gives a brief insight into urban rail energy usage by providing an adequate context in which to understand the proposed KEPIs, together with a methodology describing their application when optimising the energy consumption of urban rail systems. (C) 2014 Elsevier Ltd. All rights reserved.