... and it's incredibly doubtful that people who travel for business would opt for a 2-day train trip (one way) rather than a 4-hour flight.

Personally I love trains, and they're my preferred means of travel. But they aren't very practical, especially when you have time limitations... and at the moment they are very expensive.

OP was talking about "people commute daily between countries for work". People don't commute between London and Paris in great numbers, but the train is full of business travellers. Same as London to Manchester, or Edinburgh to Glasgow, or Edinburgh to Leeds

Obviously business people travel by train from say Glasgow to London, or London to Brussels, or Paris to Geneva, or Brussels to Amsterdam, or Amsterdam to Hamburg, or Berlin to Prague, or Prague to Budapest, or Vienna to Zagreb, or Stuttgart to Milan, or Milan to Marseille

Just because a business traveller might not do a 12 hour trip doesn't mean they don't do a 4 hour trip, and those 4 hour trips all work together in a reliable network.

Atlanta (5m) to Charlotte (1m) is 250 miles, Charllote to Richmond (1m) 300 miles, Richmond to Washington (5m) 120 miles. Population in brackets is the "urban area" population.

Seems crazy there isn't a half hourly service from Richmond to Washington DC to me, and an hourly service to Atlanta. The 6AM from Atlanta would go through, calling Charlotte around 8.30, Richmond by 12, and Washington by 1pm. The 7am would get to Washington for 2pm, etc.

Would many people do Atlanta to Washington? Probably not. Would they do Atlanta to Richmond, or Charlotte to Washington? Probably. At least with a european culture. 4 hours is deemed to be the "it's not worth flying" level.

(That said this year I've done London-Leipzig, Berlin-Geneva, Geneva-London, and New York-Miami for business on the "more than 6 hour" this year)

Sorry, I was talking about the European Union.

Where people travel via train internationally for business all the time.

This isn't really true, as it only happens in the 'Western' part of Europe (i.e. Germany, France, Belgium, Netherlands) In the 'Eastern' part (i.e. Poland, Lithuania, Latvia, Estonia, Hungary) and the 'Northern' part (i.e. Norway, Sweden, Finland) this kind of international travel happens only rarely.