This assumes that if there were no passengers on the train because they chose to drive, the train wouldn't run. This is not really what happens in reality. A train might be empty or lightly loaded for part of its journey, but can fill up quickly once it stops at a popular station, or it reaches its terminal station and makes its journey back at peak time. If the choice for me is to drive or use a train carrying fresh air, then if the train is running regardless, then driving means both my car and the train are emitting pollution, whereas if I take the train, my car is idle and not emitting pollution. This argument holds as long as the number of trains running doesn't increase as demand increases (i.e. you just cram on more standing passengers, and the extra pollution from rail travel comes from transporting extra weight). This argument doesn't hold for aircraft since airlines do respond to increased passenger demand by flying more planes, I don't think this happens much on the rail network. The more people that are persuaded to choose the train over the car, the more efficient in terms of passenger miles per gallon the train becomes, and the less traffic there is on the roads, which makes road journeys faster and more fuel economical for those who have to drive. Cars are a very inefficient use of space, which is evident from the congestion in cities resulting from a lot of people all wanting to take one car each into a small area at the same time.