The problem is that not a single one of those 10 billions buys neither a single additional vehicle-kilometre being operated [1], not a single metre of additional infrastructure, nor deals in any way with the impending maintenance backlog on the national railway network.

[1] Actually right now often the difficulty is even actually operating the currently scheduled timetable due to a lack of staff.

What they buy is a larger user base, especially of people with enough capital that they're able to make the often financially sensible decision to buy a car instead, leading to a much higher motivation to finally fix the things you mention.

So far it seems that the biggest passenger growth happened within the leisure traffic segment, and indeed it was already known from previous studies (including some smaller scale recent-ish experiments in Germany) that commuting traffic in particular is more sensitive to reliability and journey time and not that extremely price sensitive.

I'd think that commuting is simultaneously the biggest driver behind the decision to purchase a car or not, so all in all that leaves me rather sceptic on whether you'd really get people to abandon their cars en masse that way.

I'm also sceptical on the political willingness – silently and behind the scenes, some states (Hessen, I'm looking at you) have actually right now reduced their public transport budgets in order to make up Corona-induced budget holes. So the "true national through-ticketing for all public transport"-aspect of the 9 €-Ticket was nice, but beyond that I'd personally still rather see any money that can be scrounged together beyond that to be spent on service and infrastructure improvements. (And what with the staff shortages and energy price increases and possibly a recession on top we might even have to worry about even maintaining current service levels…)

But why tax everyone to subsidise the trains for the ones that use them? Even more so for tourists that barely pay taxes here and would have paid the full price anyway.

With the normal ticket prices people pay for that they use, which sounds more fair to me.

That is the basic idea of taxes: to redistribute money. The problem was a bit different though: people that need trains (like disabled people) were pushed out of trains and trams by tourists with bikes. I still really enjoyed not having to care about tickets as well.

Our roads are toll-free too and we want to encourage the use of trains.

I lived in SF before I lived in Germany. What you're describing is how BART works and it's trash.

Because we want to encourage people to take the train and discourage them using cars.

Yeah, the tax could be targeted, e.g. tax on gasoline, or on cars, with ICE cars needing to pay more (even though they're better, public transport is still the better way to prevent the climate catastrophe), but hey, the idea of any tax is probably horrifying tp the libertarians on HN...