The point of The Boring Company is to reduce the cost of tunnelling by digging narrower tunnels (designed for custom vehicles, not regular trains or cars) and using an electric tunnel boring machine.
So then why not a high speed train instead of individual vehicles? You would think the capital cost of a tunnel over it's lifetime (100+ years going by the NYC subways) is not substantial compared with magnitudes more in individual vehicle operating costs.
Not to mention trains have much higher throughput, energy efficiency, less worry over spacing via stopping distance etc over individual propulsion methods, and airports are pretty bursty in terms of traffic volume.
The answer seems pretty clear with the disdain Musk expresses of sharing public space with the average person. https://www.wired.com/story/elon-musk-awkward-dislike-mass-t... (which for me, also explains the obsession with Mars as some sort of refuge from disaster on Earth)
He's clearly focused on more cars as the solution to the urban issues that cars are causing.
This is meant to evolve into a citywide transportation system where you choose your destination at the outset. Each pod can navigate itself, switch tunnels, and wind up closer to your destination than a subway could ever hope to as there will be far more stops (multiple thousands across the city instead of < 500).
Clearly the airport express route is meant to be the beginning of something much larger.
Trains would not make sense for such a system.
If you assume an optimistic 4 people per vehicle with an optimistic 5 second interval between vehicles (which is probably not safe at the high speeds due to the frequent merges), that gives you ~50 people / minute / lane, or 3000 per hour. A single CTA 8-car train at crush capacity (admittedly uncomfortable) can carry nearly 1000 people.
Now a frequent system with high occupancy vehicles and transfers could work, but that's the same thing as a subway.
(Sorry for the excessive commas.)
Designing a transport system around some guy's phobias seems like a bad idea.
The system described only has a 2k/hour peak capacity. It's unusable as real public transport.