Well for one roads are relatively cheap tarmac is almost fully reusable and is already mostly oil industry waste.

Rail lines are much more restrictive than roads in where and how you can build them. they can't turn as sharply or have that high of an incline.

Also a good portion of most llarge city populations don't live in the city but commute. Not all of them commute to the same few areas so of course on city exits there will be a bottle neck but it thins out relatively quickly as people fuck off in multiple directions. I guess you could reduce congestion by not allowing most car in the city and having large car parks in key point outside the city. But I'm not a civil engineer or whatever deals with traffic.

Let’s consider how many people living in Berlin vs population density in rural area. It is not building it itself that is challenging but the efficiency of building such a massive network to serve a few people. It is no different than building a helipad to them. Rail maintenance is a costly work. Rails have to be replaced, tuned every now and then or trains will derail. Underlays needs to be replaced. Why bother building a multi hundred million dollar project to serve the few people in rural areas? Do you think trains does not cost energy?

Are the train going to take 1-2 passenger in rural areas every station? How do people even get to the stations? Are they going to drive? Will they question the point of driving to train station and wait half an hour for the train instead of driving to where they want? Once they get off the station, how are they going to get to their final destination? Not everything is near a train station.

There are different kinds of people living in different parts of the country so just to say no cars for everyone is exactly the same as saying people should all drive and ban all public transport. We need a mixture of both for everyone to get to work efficiently. In all the bigger cities, public transport is a crucial component. Not so much in less densely populated areas.