electromagnetism in Porto's Metro system, an overground railway which connects urban and sub-urban areas. Unprocessed field recordings of the electrical activity inside the rail system. 2014
"Electrified rail propulsion motors move the trains by converting traction currents into kinetic energy. Depending on the electrified supply, time-varying AC or DC current, magnetic fields emanate from the overhead catenary cables and fourth rail supplies as the trains and subways move along the rails. The magnitude of the magnetic field emissions are related to the amount of current required to move the trains, which depends on the number of cars and passengers, number trains (rush hour), and proximity (distance) to the tracks. The ferromagnetic (steel) mass of the moving trains, elevators and vehicles also generate quasi-static DC magnetic fields by perturbing the Earth’s magnetic field."