I'd say that 4-6 hours is often the cut-off point for total travel time city-to-city -- provided that the train is fast.

I and a friend once started at Stockholm Central station to Malmö Central Station at the same time. I boarded a bullet train; he took the bus to the airport ... We arrived at the same time six hours later.

I've been looking at travel in Japan recently. Between Hiroshima and Tokyo, the bullet train (fast Shinkansen) and flight are both about four hours.

A bullet train is much more comfortable than flight within these limits because there are so fewer steps: no travel to/from airport, fewer tickets to book and often no check-in or security checkpoint.

Yeah fair, having experienced Japanese trains I fully agree about the comfort. I'm not arguing that we shouldn't build high speed rail and try to shift more travel to trains instead of aviation either to be clear. I love trains and I especially love Shinkansen style trains, but aviation still has a big edge in big parts of the world and on some journey lengths.