Just make it mandatory (basically tax) and remove all those garbage ticket systems, people checking the tickets etc... Getting rid of so much overhead
No train system in the world has all of that, which in turn means they need more money and so free riders harm the system's ability to get people where they want to go.
By everywhere I include farms in the middle of nowhere as that farmer has a relative somewhere who wants to visit and the lack of great transit pushes that person to a car and then opposing transit subsidies. By getting some money from riders there is at least some inventive to make good compromises.
I believe there are taxes in Germany that are zweckgebunden (legally bound to be used for a particular purpose), but the Soli isn't one: there were political justifications for its introduction, but it goes into the general fund and the government doesn't have to use it for those purposes.
As a random example here is a tax for sparkling wines. No, not import custom fees. A tax.
So why bother with the overhead in the first place? Make it free for everyone to use to hop/on/off as they please, like Luxembourg has done.
Taxes and public subsidies for infrastructures are especially powerful on networks because the output service is spread in many cross dimensional layers of the nation.