If we look past carbon emission, we can see that the value is much bigger.
You mention that the rail industry is already substantially subsidized. What you did not mention is that the automotive industry is as well. Roads are paid by state and republic. Car purchases are often subsidized via a flat return by the republic (which primarily benefits the rich). Petrol is also subsidized.
Now all these automotive subsidiaries benefit only the people who own cars, and much more the wealthy ones. In Germany, only 43 million private households own a car. That's just roughly 50 %.
But if you get people to choose public transport over your auto, then this benefits everyone. Better air quality, less noise pollution, less streets and more space for pedestrians. Anecdotally, a close neighbourhood removed some parking spots in favour of space for local cafes and public benches with plants. Removing just two parking spots give enough places for 10 people to sit together and enjoy a sunny Sunday -- which otherwise would be dead space occupied by the cars of two or even one household.