I'm not saying that countries with lots of rail are wrong. I'm saying that passenger high speed rail doesn't make sense in the US (at least, not outside of the northeast). We're too spread out.
Air travel is a problem when it comes to emissions because there really aren’t any viable alternatives, where as you could pretty much sub in nuclear + renewables for anything else energy/transportation wise and the math makes sense after a large investment. Knocking overland air travel out and replacing it with high speed rail cuts down a ton of air travel, and replaces it with something that is almost trivial to run on cleaner energy.
Another part of that comment you were responding to dealt with comfort and convenience, and having ridden HSR in Japan and Europe, I much prefer it to air travel. Air travel sucks and it’s only getting worse as fuel costs more. If you’re not flying business, you’re basically treated the same way livestock is treated, with a multi-hundred dollar or thousand dollar price tag to add to the insult. I’ll vote for my tax dollars going to HSR all day, “boondoggle” or not. I remember in Massachusetts the big dig was marked as such, but man has it worked. Tunnels under the city where you can drive 45 rather than elevated roads or no roads or surface streets, yep it was expensive but it’s so much better. I’m willing to bet with my tax dollars and ballot measure votes HSR will be the same.
And unfortunately carbon offsetting is rather unreliable (companies buying offsets don't have any incentive for the offsets to be actual savings). I'd be wary of climate change solutions that consist of continuing business as usual and assuming that decarbonation will happen in some other sectors. I also suspect the 20% price increase estimate works for a specific price of carbon, but that price could increase if carbon offsets started becoming widely used (and the demand increases).
Anyway, I don't know what's the right way for the US to reduce travel emissions, whether that's electric coaches, cheaper rail, taxation to increase prices and reduce demand... But I don't think the solution can be "just keep planes and add carbon capture".