What about the railway staff and/or police who have to go onto the track to get the person off, @LoopyLou? Do you want to run the risk of one of them tripping (or being pushed by the trespasser) onto the live rail, and dying?
For the trespasser to be removed, the track has to be made into a safe working environment for the people going after them - this means turning the power off - end of.
Also, as an earlier poster said, if the trespasser ends up in front of the train, deliberately or by accident, the driver is highly unlikely to be able to stop the train in time - trains can take over a mile to perform an emergency stop - and drivers who have hit trespassers on the line are traumatised - some never work as a train driver again. Better some inconvenience to you, than that trauma for a train driver, imo.
Not to mention the fact that, if the trespasser is hit by a train, or electrocuted by the third rail, your delay will be much, MUCH longer, as they organise removal of the body, and get the Rail Accident Investigation Board investigators to look at the scene, to see why the incident happened.