It isn't necessary to think of electrical current (electrons) as flowing back to its original location through the Earth, the ground, or a grounded rail. By connecting the return side of a circuit to a ground, you allow electrons that had been boosted to a voltage (energy state or electrical potential) greater than ground by a power source (such as a battery or transformer) to return to ground potential - the energy state where they were initially. In the process of moving through the circuit, the increased energy in the electrons may have been used to turn a motor which moves a train, and the electrical energy is converted to kinetic energy (energy of motion). After that energy transfer occurs, the energy level of the electrons returns to its initial value - the ground potential.
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