I once saw a receipt for a cross-country train ride in the late 1950s and was shocked at the price. It was significantly slower and more expensive than a similar plane ticket today.
That tends to be true now as well for a variety of reasons. I'd be more interested in the cost then between both modes of travel.
Yes, in the USA Amtrak tends to be quite a bit more expensive than the equivalent trip by air. And far slower. Maybe excluding some of the east coast commuter routes.
Very different experience though. Fast and deeply uncomfortable vs long and chill. I'd prefer the latter when feasible but it obviously isn't always.