Barcelona's metro system has implemented an ingenious approach to energy sustainability: regenerative braking technology. When trains decelerate, the kinetic energy that would normally be lost as heat is captured and converted back into electricity. This recovered power feeds directly into the grid, reducing the system's overall energy consumption and operational costs. The mechanism works through electromagnetic induction, where the train's braking force triggers generators that transform motion into usable current. This innovation reflects a broader trend in urban infrastructure—maximizing efficiency through smart energy recovery. It's particularly relevant as cities worldwide race to cut carbon footprints while supporting growing transportation demands. Similar technologies are being explored in electric vehicles and renewable systems globally. Barcelona's solution demonstrates how retrofitting existing infrastructure with regenerative capabilities can deliver measurable environmental and economic benefits without massive overhauls.