Which is cheaper: commuting by car or train?

Commuting costs an arm and a leg, however you do it. Anyone forking out a small fortune for a rail season ticket would be forgiven for thinking that nothing could be more expensive. Meanwhile, those handing over half their salary at the petrol station every month to pay for their drive to work must surely be convinced that they face the highest costs.

But who is right? Which form of commute is cheaper?
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