Electric trains do not carry fuel on board in the traditional sense, as they do not have internal combustion engines. Instead, they receive a continuous supply of electricity from an external source, such as overhead catenary wires or a third rail. This electricity is generated at centralized power stations (which may use coal, gas, nuclear, or renewable sources) and transmitted through the grid to the railway's substations. However, some "bi-mode" or "hybrid" trains carry diesel fuel to operate on non-electrified sections of track. Purely electric trains are significantly more energy-efficient than diesel trains because they don't have to carry the weight of a massive fuel tank and can utilize regenerative braking, which turns the train's kinetic energy back into electricity to be fed back into the power lines or stored in batteries. So, while the train itself is "fuel-free," the system as a whole relies on the fuel mix of the local power grid to generate the energy the train consumes to move.