As others have said, you are describing a totally imaginary world where money is the only cost. “Artificial” is doing all the work. But the very investments you’re describing are “artificial”, and more than that, they require constant spending to maintain. Why should cost at point of use be the only artificial incentive? What about the environment created by those investments? The quality of roads, the cleanliness of the train? Your distinction is contrived in service of your predetermined conclusion.

But when there are multiple competing forms of transit, or high externalities caused by use (or overuse) artificial disincentives are optimal. As an example, if you have access to both a car and a train, and the car pollutes less than the train, some artificial incentive to only use the car when necessary and to use the train otherwise is actually optimal.

You're also, it seems assuming that investment is a one-time thing. Once an investment has been made in a train line or a car, you still need to afford maintenance over the thing over the lifetime of the thing. Including the opportunity costs of doing other things instead.

Taking up space, degrading public infrastructure, polluting the air, and killing pedestrians are real ongoing costs of transportation. The cost does not magically end at vehicle purchase.