A train is hurtling down a railway line. It's out of control and the brakes have failed. Ahead, five people are tied to the track and apparently face certain death. You, however, are standing by the rails and have a chance to save them.

This is the story of an obese gentleman, runaway trains, killing and saving lives. The programme examines moral philosophy and our deepest moral intuitions through a single famous thought-experiment: would you kill the big guy?

In the first programme we saw how most people believe its acceptable to divert a runaway train away from killing five people who are tied to a track, even if by so doing, the train would crash into and kill one other person.

But the majority of people do not think it’s acceptable to stop a train by throwing a large man in front of it - even if this would save five lives.

Why does this apparently arcane philosophical puzzle matter in the real world?

Well, the politician, the lawyer, the doctor and the soldier all have to make decisions about life and death - about who should live and who should die. The Trolley Problem casts light on when their actions are moral, and when they’re not.

There have also been dozens of studies on popular intuitions in these cases.

The BBC recently conducted an online poll in which 65,000 people took part. Neuroscientists and psychologists have also jumped on the tramwagon. In Harvard and Princeton, brain scans are showing which bit of the brain is being used in the different scenarios.

This two-part series, presented by Steve Evans will explore the tram puzzles - to see what it tells us about what kinds of moral creatures humans are. Show less