Boston to NYC or NYC to Washington are relatively short runs (under 4 hours or so) as well. And they're in a very populated corridor in which IMO the driving isn't very pleasant. (Plus they can take you downtown to downtown which is often an advantage.)

Train gets a lot less interesting as an option as the distance increases. I have taken Amtrak from Boston to Washington and it's definitely doable but it's not really especially competitive with flying .