I'm not sure you've fully accounted for the full carbon reduction here, which is not just the replacement of cars from the road but also the transition to a car-free lifestyle.
It gets people out of their cars because it's cheaper than petrol. When train tickets (in other countries) cost more than the cost of petrol for the journey, commuters who already sank the cost of buying a car + insurance ask why they should bother paying a premium for the train ticket when the cost of petrol will be cheaper.
Only after people transition to taking the train in the morning, do they then start to ask questions like "do I get value out of paying for insurance on a car I don't drive?" and start to make decisions like selling their cars. In this case the carbon reduction is not only the carbon reduction of the trips that would have been taken by train, but the carbon reduction from the use of the car altogether, and future car maintenance and replacement.
It gets people out of their cars because it's cheaper than petrol. When train tickets (in other countries) cost more than the cost of petrol for the journey, commuters who already sank the cost of buying a car + insurance ask why they should bother paying a premium for the train ticket when the cost of petrol will be cheaper.
Only after people transition to taking the train in the morning, do they then start to ask questions like "do I get value out of paying for insurance on a car I don't drive?" and start to make decisions like selling their cars. In this case the carbon reduction is not only the carbon reduction of the trips that would have been taken by train, but the carbon reduction from the use of the car altogether, and future car maintenance and replacement.