A car IS a greener form of transport than a train - even an electric train, given the manner in which the power is generated.

However, a train carrying 600 people is greener than 600 cars each with only a driver.

Beyond this, no comparisons can be made, since the majority of commuters do so by car, on their own, and as a result, many trains (OK, not the London ones!) run only part-full.