Background Rail transport is often an environmentally friendly option: it is, for example, between three and five times more energy efficient than most road transport. However, there is still room for improvement and rail transport must contribute further to the EU’s effort to reduce CO2 emissions. Thus, one of the principal objectives of European railway policy is to achieve a 50% improvement in energy efficiency over vehicle lifespans by the year 2020. Among the different ways to reduce the CO2 footprint of rail transport, the recovery of braking energy is considered as one of the most effective. It is currently standard practice to fit railway traction units with regenerative dynamic braking systems. Thus, energy generated by a braking train can be partially reused by other accelerating trains. For example, in a dense metro system, around 60% of braking energy can be recovered. However, in direct current (DC) systems (light rail, metro, or suburban, regional and main lines in certain countries), due to the unidirectional flow of power in the diode rectifier substations, the unused braking energy (40% in a dense metro system) cannot be sent back to the grid and is burnt in resistors. Objectives The RE-USE project aims to demonstrate and validate, in terms of energy efficiency and the reduction of CO2 emissions, a new innovative technology called ‘the Reversible DC Substation’. This new technology will make it possible to send that part of the braking energy that is usually burnt in resistors, back into the grid, resulting in a significant reduction in CO2 emissions. The validation and quantification of the reduction in the environmental impact will be facilitated by the full-scale demonstration of the technology on a metro line in Milan. Therefore, the three main objectives of the RE-USE project are to: Reduce the environmental impact (CO2 operating footprint) of the rail sector, especially public transport in urban areas. The project will quantify the energy saved, and thus the CO2 reduction achieved by the new Reversible DC Substation. The energy system performance will be optimised for the 1500V DC demonstration system used on the Milan metro line; Demonstrate that urban guided systems equipped with the Reversible DC Substation technology can be applied universally to all DC rail systems (600V, 1500V and 3000V); Keep the project’s environmental footprint as low as possible. The design and manufacturing of the Reversible DC Substation will have a minimum impact on the environment, as the process considers all aspects from eco-design and recycling to the safe use of chemicals. The design will respect the environmental and sustainable development criteria required to qualify for Environmental ISO 14021/14025 certification. Expected results: A reduction of 15% in energy consumption during traction. This represents a saving of 1.3 GWh for the metro line tested in Milan. Note that with current technology, the energy greatly increases the temperature in stations, tunnels and rail cars. The removal of this heat loss will also reduce energy consumption for cooling; The identification of a technical solution for saving energy through braking energy regeneration, which can be applied universally to all the DC-fed systems; The development of a product with a low lifecycle cost (LCC) and a minimum environmental impact, in order to obtain the ISO 14021/14025 certification. A Life Cycle Analysis of the product will also be carried out.