In the US, Acela--which is as good as it gets in the US--is still a full day trip from Boston to Washington DC while a flight is less than two hours. It's not as just-roll-up-to-the-airport as it was pre-9/11 but it's still something you can do to catch a mid-morning meeting.

The train is pretty much a full day. I've done it when schedule allowed but going down the day before, working on the train, and spending the night in a hotel as opposed to doing an out and back in a day doesn't work for a lot of people.

Of course, shorter segments of the Northeast Corridor work pretty well by train. I pretty much never fly Boston to NYC unless I'm under unusual constraints.