Here is a very clear discussion by T.Campbell of two classic utilitarian dilemmas.  A runaway trolley is heading towards twenty men on the track.  You have the points switch in your hand.  In a siding there is one man lying ona another trolley.  If you flick the switch you kill the one man.  If you do nothing, twenty men die.  This case is discussed in Michael Sandel's introductory lecture in his justice course at Harvard.