When a train passes through a tunnel, the air in the tunnel is squeezed by the train and follows it. A negative pressure region forms behind the train, drawing fresh air into the tunnel and pushing the existing air out. This phenomenon is known as ‘piston effect’. In this project, Manchester Simulation was commissioned to conduct a CFD-based feasibility study to assess the effectiveness of harvesting energy from the piston effect using a wind turbine in a typical tunnel. CFD simulations were conducted for a single 5-car Class 802 train, passing through a tunnel. The CFD results captured the flow physics and estimated the piston effect for different scenarios. Air velocity in 3 different locations within the tunnel were obtained and by utilising experimental data, the maximum potential power output in Watts was estimated for each location. Due to the success of this project, the UK Research Council funded the continuation of this project for our team to develop a novel vertical-axis wind turbine (VAWT) to utilise this piston wind and to be deployed in the UK.