European rail companies have claimed rail can be competitive for up to 4 hours, which matches my impressions of these journeys. London-Amsterdam and London-Frankfurt (which DB were talking about running at one stage before the problems with the new ICE models - don't know if that's still planned at some point or not) make sense, but that's about the limit tbh.

That feels about right and would seem to be consistent with both my own preferences and my observations on the Acela in the US Northeast Corridor. The two halves of the corridor (north and south from New York City) are heavily traveled by train and a lot of people prefer the ~3.5 hour trip to flying.

However, my impression is that relatively few take it all the way from Boston to Washington DC. It's still about the same 1 hour flight that the shorter legs would be by air but now it's basically a full day of train travel. I've done it when I've just been happy to work on the train but it doesn't really make a lot of sense most of the time.

It's the highest speed trains. There is a speed meter on one side of the carriage. The cruise speed is beyond 300 km/h.

Yes. But that's not the average given stops etc.

Even with a train service like the Shinkansen, getting from Tokyo to, say, Hiroshima is over a 5 hour train ride for a distance of a bit under 1,000 km.

I like taking trains in Europe and Japan but there's an upper distance limit for when they make utilitarian sense. (For London to Brussels or Paris though, it's hard to imagine why I'd want to fly even if it were a little cheaper.)