There is nowhere to regenerate the energy to (no battery, and no overhead line to feed the energy to where it can power other trains). Instead, diesel-electric

There is nowhere to regenerate the energy to (no battery, and no overhead line to feed the energy to where it can power other trains). Instead, diesel-electric locomotives have dynamic braking. The motors are reconnected as generators as is the case with regenerative braking, but the electrical energy is fed into what is basically giant heatsink (fan-cooled resistor grids). As for eddy currents, I imagine there would not be enough interface area between the rail and the electromagnetic element to produce enough braking force against the huge amount of kinetic energy a heavy moving train has. Dynamic braking already solves the problem of saving wear from friction braking, consider it similar to engine braking a car.