According to
Amtrak's "Travel Green" website, they state that Amtrak is 34% more energy efficient than flying, and "up to 73% fewer emissions." The emissions claim is unsourced and has an up-to, while the 34% more energy efficient comes from the 2021 U.S. Department of Energy Data Book. That publication has a warning on both graphs that talk about it (
the main one, not including power distribution losses, and the
secondary one that includes power distribution losses):
"Great care should be taken when comparing modal energy intensity data among modes. Because of the inherent differences among the transportation modes in the nature of services, routes available, and many additional factors, it is not possible to obtain truly comparable national energy intensities among modes. These values are averages, and there is a great deal of variability even within a mode."
Given that rail isn't leaps and bounds more efficient than air on the sourced claim, I think it's reasonable to infer that there are instances where particular Amtrak trips in particular classes of service may be less efficient than particular air trips in particular classes of service - where the mode used on Amtrak is particularly energy-intensive (say, a sleeper compartment which uses diesel engines and is not terribly dense, and with current dining services also has to account for the diesel used by the dining car) and the mode used in the air is somewhat less energy-intensive than the average (maybe not domestic first, but domestic coach on a mainline jet on a longer flight is likely more energy-efficient than the average air mile, since the average also includes small turboprop planes, higher passenger-mile emissions for short hops, etc.)
It's important to note that I'm not saying that rail travel
in general is less efficient than air travel
in general (clearly rail travel is better on average,) and rail certainly has a clearer path to getting to low/zero emissions (electrifying the rail network moves emissions to the electric grid, which looks to be moving to zero-carbon by mid-century in many places.) With how the landscape looks today, though, I'm not sure the argument is as strong for the efficiency of traveling in a sleeper car pulled by a diesel locomotive over air travel, particularly mainline jet travel. I'm not convinced that jet travel is significantly better, either, just that they're probably in the same ballpark.