Assuming the fat man was fat enough to stop a trolley, in order to be that fat he would have to be quite round, meaning he can be rolled over on his sides, with his arms tucked into his fat so that he has a very smooth surface. This smooth surface will minimize friction, combined with the fact that rolling something round rather than pushing it requires much less force to overcome the static coefficient of friction, allowing a mere mortal like ourselves to roll him over onto the tracks when the situation calls....take that physicist.