For those of us who live and work in big cities – well over half the world’s population today, a figure projected to increase to 70% by 2050 – metro railways are an ever-present part of daily life. Today there are 148 cities with metro operations carrying over 150 million passengers per day, with about one-third in Asia, one-third in Europe, and the rest split between the Americas, the Middle East and Eurasia. Metros are critically important assets for the world’s cities in order to meet the huge challenges of maintaining urban mobility in the coming decades. In this article, Rick Eagar, Russell Pell and Philip Webster discuss the five key challenges for urban railways to meet future mobility needs.