If the power provider (I only tested with 1) loses all electricity, nothing changes in the electric locomotives - they still can drive as much as you want, their "fuel level" remains full. This is too much cheating. By contrast, if the power provider itself is removed, the locomotives lose spend their "fuel" very quickly (as expected); I thought the same should happen when it's depowered.
Found under Factorio 0.17.36.