What highway?
What is the "safe" capacity of just one lane on any highway or street?
Safe means 2 seconds gap between vehicles in that lane of traffic.
30 vehicles per minute, 180 vehicles per hour, 4320 vehicles per day, 1,576,800 vehicles per year.
Single occupancy vehicle 1,576,800 passengers per year
Double occupancy vehicle 3,153,600 passengers per year
Quad occupancy vehicle 6,307,200 passengers per year
Octo occupancy vehicle 12,614,404 passengers per year
https://media.amtrak.com/wp-content/...-Ridership.pdf
Pre Covid, Amtrak ridership on the NEC was around 12.5 million passengers per year.
In no way am I suggesting a single highway lane will have as much capacity 24-7, but I am suggesting it could.
Whichever you decide to build; planes, trains, or automobiles; capacity issues arise. New lanes, new tracks, larger stations, larger parking garages, so many variables it is impossible to list all of them, come into play. Each solution has it own costs, without considering costs it is impossible to state which will be cheaper for the average intercity passenger.