I have some published LCAs at home, but the gist is that most US studies:
-assume improving existing infrastructure instead of new right of way and new track, which limits speed to ~125 mph
-calculate efficiency ignoring embeded energy/costs (fuel per passenger mile, instead of complete LCA)
-are based on unrealistic track usage. At some capacity a track can't hold anymore trains and you need a second track, while airspace for additional planes is flexible and almost free. Also the faster the train, the further they need to be spaced out, which typically reduces capacity.
-price subsidies such that ticket price per passenger mile is roughly $0.45 rural - $0.55 Urban, which matches airfare for shorter flights, but not longer ones. (500+ miles)
-that pricing is targeted because it correlates roughly to the service window where time of travel is roughly competitive with air travel, taking into account check-in time.
I can't find anything online that suggests air is ever cleaner than rail. The discrepancy is a factor of 20¹, so it would be surprising if circumstances overcame that, unless you are traveling across an ocean.
For idealized shorter range distances (300~350 miles) I've seen claims of 2x or 2.5x, sometimes up to 5x, but that's using full LCA, not just fuel per passenger mile. The embedded energy in ROW and track are significant, and it doesn't scale well (capacity wise) compared to air travel.
In favor of hsr, but still with some big assumptions and limited distances. The fuel factor is huge, but still relatively comparable between modes, and it's all heavily dependent on % of empty seats.
95% is extreme, but it's usually assumed to be in the 55-65% range for generic civil infrastructure, not hsr specific. Compared to that hsr is heavy on concrete and steel, so would be higher.
Embedded rail infrastructure costs are calculated based on near track capacity (amortized over 30-50 years, which is generally correct for civil projects), and typically near train capacity. Empty trains are much worse than empty airplanes, and if ridership isn't projected accurately pulling a train from a route doesn't reclaim any embedded costs of the track or row, while airline infrastructure can be completely reallocated. Multiply a miscalculated capacity by 50 years and it turns into a big number.
I'm about 2/10 so far on finding these papers I'm looking for today, but this one at least addressed ridership implications for the California project in terms of LCA.
And here's one comparing air to hsr in Europe, showing only 3%-20% gains by shifting passengers from air to rail, based on existing infrastructure, but using LCA assuming a %50 recyclable rate for rail.
Reason probably is that, at long distance, lower air resistance at height starts to count.
Also, it’s easier to move planes around than to move rail tracks around, so if a destination is popular for only a short time (say for the Olympic Games, or for the hajj (https://en.wikipedia.org/wiki/King_Abdulaziz_International_A...), it may be energetically cheaper to build and (basically) discard an airfield than to build and (basically) discard a long rail track.
>Reason probably is that, at long distance, lower air resistance at height starts to count.
This isn't as big a factor as you might think. While older aircraft were optimized for design/cruise altitude, modern jets are designed to account for the imposed altitude restrictions and speed limits of departure and approach. They also have a much wider/efficient operating envelope, and due to improved logistics, are dispatched, loaded, and operated more appropriately wrt to flight plan profiles.
Modern passenger jets are incredibly efficient. 1000-mile rail ROWs take a ton of resources to maintain and capacity does not scale well, especially as speed increases.
