Kenan Malik in Pandaemonium:
When faced with the question of whether or not to switch the runaway train, most people, unsurprisingly, say ‘Yes’. Now imagine that you are standing on a bridge under which the runaway train will pass. You can stop the train – and the certain death of five people – by dropping a heavy weight in front of it. There is, standing next to you, an exceedingly fat man. Would it be moral for you push him over the bridge and onto the track? Most people now say ‘No’, even though the moral dilemma is the same as before: should you kill one to save the five?
Or consider a dilemma first raised by Peter Singer forty years ago. You are driving along a country road when you hear a plea for help coming from some roadside bushes. You pull over, and see a man seriously injured, covered in blood and writhing in agony. He begs you to take him to a nearby hospital. You want to help, but realize that if you take him the blood will ruin the leather upholstery of your car. So you leave him and drive off. Most people would consider that a monstrous act.
Now suppose you receive a letter that asks for a donation to help save the life of a girl in India. You decide you cannot afford to give to charity since you are saving up to buy a sofa and bin the letter. Few would deem that to be immoral.
Again, there seems to be no objective difference between these two cases. Yet to most people they appear unquestionably morally different.
More here.