Trains are just an anecdotal example - today we also have "dial a ride" services, where you can call for a bus (during operating hours).

Once your population density goes below a certain threshold, individual traffic isn't a problem anyway. This isn't the case, however, with most of Europe.

To be honest, I'm not so sure where most of Europe falls on the "viability of public transport" spectrum. But that dial a ride service sounds like a nice alternative for otherwise too rural regions.