An appropriate driver-assistance in order to use regenerative braking effectively is significant as a feasible method of a smart power management for energy-saving and economical solutions in sustainable electric railway operation. It is substantially significant to suppress braking force in high speed so that the braking force is less than corresponding traction force in the field-weakening speed region so that the whole braking force can be supplied by the regenerative electric brakes without active use of ordinary mechanical brakes. This power limitation in high speed is also beneficial to reduce the probability of regenerative braking cancellation caused by occasional high voltage at pantographs. If one limit the braking force, the total traveling time is increased, but this supplemental running time is relatively small, when the limitation of the deceleration is only applied in high speed and this supplemental time can be absorbed in realistic time margin, which is the difference of the initially scheduled conservative running time and actual one. Authors propose this type of the fully used electric braking pattern as 'constant-power braking'or 'power-limiting braking'and for realistic application to existing rolling stocks, they also propose a minor variation called as discrete approximate constant-power electric braking. The train driver must start braking operation considerably earlier than conventional ordinary train operation, and the braking force must be precisely controlled depending on the actual train speed timely. Since this driving method is, therefore, inherently more complicated than conventional one, a smart onboard driver-assistance or automatic train stopping control is requested for the realization.