The 1,000 mile rule is pretty widely quoted and is not too controversial. Reasons for it are:
1. Speed (physical limitation). Due to reduced overhead (time getting to and at airport), trains are faster or equivalent to air travel for short (<500 mi) routes, and are not much slower for medium (<1000 mi) routes. Whereas for cross country (~3000 mi) routes, even a high speed train would take a couple of days for a trip that could be made in 10 hours by plane (incl. overhead).
2. Infrastructure costs. Air travel infrastructure is proportional to the number of passengers and constant with respect to the travel distance (since each passenger occupies 1/200th of a gate at two airports for about an hour, regardless of the travel distance. Rail travel infrastructure is proportional to the distance traveled and relatively constant with respect to the number of passengers. These combine to mean that shorter trips with more passengers are more competitive by rail, and longer trips with fewer passengers are more competitive by plane.
1. Speed (physical limitation). Due to reduced overhead (time getting to and at airport), trains are faster or equivalent to air travel for short (<500 mi) routes, and are not much slower for medium (<1000 mi) routes. Whereas for cross country (~3000 mi) routes, even a high speed train would take a couple of days for a trip that could be made in 10 hours by plane (incl. overhead).
2. Infrastructure costs. Air travel infrastructure is proportional to the number of passengers and constant with respect to the travel distance (since each passenger occupies 1/200th of a gate at two airports for about an hour, regardless of the travel distance. Rail travel infrastructure is proportional to the distance traveled and relatively constant with respect to the number of passengers. These combine to mean that shorter trips with more passengers are more competitive by rail, and longer trips with fewer passengers are more competitive by plane.