Let's assume 4000 vehicles/hour is the whole system, meaning 2000/h per direction. This is 1.8 seconds between vehicles moving at 250 km/h or 70 m/s. 126 meters between vehicles moving 70 m/s does not give much room for error.
This may be doable in theory but is the system really able to handle sustained volumes in such speed and interval? How to efficiently on and offload so many cars? Just like a rifle can shoot 700 shots / minute but there is only 30 rounds in the magazine.
No need to assume.
From the TBC FAQ:
Loop is targeting 4,000 vehicles/hour at 155mph (250km/h) for each Main Artery Tunnel. If a second tunnel is added below the first, this value doubles.
(Emphasis mine.) A “practically unlimited” number of branch tunnels, entrances, exits and stations will feed the main tunnel:
Unlike a subway, there is no practical upper limit to the number of stations that can be built along the tunnel route, as stations can be as small as two parking spaces.
The electric vehicles descend into a "spur" or side tunnel before quickly merging into a Main Artery Tunnel. Since stations require such a small footprint, they can be easily integrated in busy city-centers, parking garages, and residential communities. The high density of stations will help reduce congestion by distributing traffic across many access points and by providing more convenient entry and exit locations. At the same time, larger stations can be built to increase ridership capacity to a specific central location.
There is also no practical upper limit on the number of layers of tunnels, allowing the Loop system itself to always have capacity available to service each station.
How much is 4000 vehicles/h? With full 5 persons per vehicle the theoretical max capacity is 20000 people/h or 10000 one way. This is a bit more than light rail, and appr. 10% of an efficient metro line.
On the plus side TBC doesn't really need stations which are the most expensive part of an underground rail line. It's also flexible for the passengers if they can continue to their final destinations.
More from the FAQ:
How is Loop different from a subway?
Loop is an “express” public transportation system and more resembles an underground highway than a subway system. Through the use of a Main Artery Tunnel with side tunnels for entry/exit, passengers travel directly to their final destination without stopping.
As an example, if a train-line had 100 stops, the train would typically stop at each one, so the trip from Stop 1 to Stop 100 would be long. For Loop, passengers would travel directly from Stop 1 to Stop 100 without stopping at the intermediate stations.
Equivalently, a Loop vehicle’s average speed is close to its maximum speed, while a train’s average speed is much less than its maximum speed. A subway car might be capable of traveling 65 mph, but its average speed might be 20 mph, decreasing further as more intermediate stations are added.
Additionally, autonomous electric vehicles are generally faster than conventional subway cars (150 mph vs. 65 mph), and, because of Loop’s architecture of high-quantity, small-footprint stations, Loop can get passengers physically closer to their final desired destination.
https://www.boringcompany.com/faq The downside is extremely expensive rolling stock. And then there is the underlying physical fact that it is just awfully expensive, energy- space- and moneywise, to move a handful of people in a 1 ton metal shell.
Compare the efficiency of a big heavy rail car when it carries only a handful of passengers, or is even empty, but must maintain its schedule. Needing to waste energy coming to a full stop at each passenger’s station, then start up again. More noise pollution, as well as oil and grease deposits from the equipment.
In the age of COVID, consider the benefits of traveling in smaller groups, being enclosed for a shorter time, plus the ability to disinfect each car quickly and automatically between rides, on a side track. The Loop’s flexible scheduling and smaller stations means fewer people crowded together waiting to board a train at one specified time. Fewer passengers exiting and entering through the same limited door-space.
“Passenger throughput” is but one measurement of many needed to judge a Loop’s worth.
Edit:
Forgot to mention that weather like snow, freezing rain, flooding, or heat high enough to warp metal rails will not stop a Loop, while it can bring trains to a standstill.