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The determination of the mass and the width of the Z boson at CERN's LEP accelerator, an e + e storage ring with a circumference of approximately 27 kilometres, imposes heavy demands on the knowledge of the LEP counter-rotating electron and positron beam energies. The precision required is of the order of 1 MeV or 20 ppm frequency. Due to its size the LEP collider is inuenced by various macroscopic and regional factors such as the position of the moon or seasonal changes of the rainfall in the area, as reported earlier. A new and not less surprising eect on the LEP energy was observed in 1995: railroad trains in the Geneva region perturb the dipole eld. A parasitic ow of electricity, originating from the trains, travels along the LEP ground cable and the vacuum chamber, interacting with the dipole eld. An account of the phenomenon with its explanation substantiated by dedicated measurements is presented.to be submitted to Nuclear Instruments and Methods