I.M. have rather fast response, and by using V/f control in closed loop configuration you'll be able to control speed back to permitted range in a matter of seconds, hence you'd be able to get withing the permitted speed within a minute. on gradient certain leeway is allowed but most LP's have enough road learning of section to control the train well in time to reduce speed in advance, hence Over speeding rarely happens; such measures also help save energy, hence advocated by everyone, from SrEE TRO onwards till LI's