This topic comes up periodically on HN and it typically ends in the same points:

1. Your entire country (UK) is smaller than 11 of our 50 states. The several hundred miles between London and Edinburgh pass through many smaller cities and towns, between which people travel. Between SF and LA is a pea soup restaurant and a handful of midsized, mainly agricultural settlements that can't be gotten around without a car (or pickup if we're being specific). SF/LAers aren't going to Stockton, or vice-versa.

2. Your entire continent, minus Russia, is smaller than the lower 48 states (2.5 vs 3.1 million sq miles), with a much higher population density and, again, actual places every 10 miles or so. I've bicycled across the US, there are stretches bigger than Western Europe that are full of nothin in particular.

3. Look at some population density maps. The US is light on corridors of sufficient size & density to make much sense for HSR considering the space between population centers: the Northeast (already served by HSR) and my native Chicago-Milwaukee-NW Indiana corridor (already served by commuter rail). Maybe Florida where they're actually building rail, surprisingly. Detroit area, maybe throw Ohio & Pittsburgh in (Amtrak partly covers it). The eastern 43% of China's land area (~1.6 million sq miles) contains 94% of the people (~1.3 billion people) See: https://en.wikipedia.org/wiki/Heihe%E2%80%93Tengchong_Line
In Japan you can basically draw a single, ~1400 mile line from Hokkaido in the north to Kyushu in the south that hits most of the population centers. Actually, they did that for you, it's called the Shinkansen and it is awesome.

4. This is actually the main point but I thought I'd sh!t on passenger rail first. The US has the largest and most extensively used rail network on the planet earth. We just use it to move freight, which doesn't breath, eat, use the lavatory, attempt to explode the vehicle it's on, or (to stay my sarcasm for a moment) complain when it's delayed.
I haven't done the numbers here, but that probably offsets a lot more GHG/oil than replacing airports with passenger rail would.

5. To dovetail on earlier points, the US is largely defined by invisible cultural borders. To a disturbingly large extent, city people travel between cities and country people travel around the countryside. Each are often actually afraid of the other places. City people talk about 'flyover country' or make jokes about rednecks, incest, the banjo from Deliverance. Country people talk about 'inner city violence'. This is a newer, still emergent phenomenon and only partially true (hence it's placement last on my list).

All this said, I love Amtrak and ride when I get a chance to take a little time off and enjoy the scenic route. You see the country from a grounds-eye view that you can't from the air, and it's pleasant to walk around the cars and talk to people you otherwise wouldn't (I once chatted with Seth Green's mom - or a woman convincingly claiming to be Seth Green's mom - on the City of New Orleans). On the longer routes eg Empire Builder you can buy one ticket and stopover at interesting places along the way and get back on tomorrow's train with the same ticket. I'm looking forward to taking the train from Anchorage to Denali next year. But I'll be flying home at Christmas, with only 5 days off.