Okay, the Loop proposal is absolutely bonkers, so let's go over why their "benefits" aren't so:

> Make the tunnel smaller.

The actual project costs of tunnel bores for the sizes of subway tunnels run closer to linear in diameter, not quadratic as you might imagine. A lot, indeed most of them, of tunneling costs aren't about the volume of fill you're extracting.

Furthermore, there's a massive cost to making them smaller: you're now limiting the interoperability of tunnels with other infrastructure. It becomes bottlenecks that requires things like unloading cargo in a warehouse and reloading them onto different infrastructure, or suffering reduced throughput on the existing infrastructure because you don't have as much capacity in the existing infrastructure. In the US, the freight trains have basically standardized on Plate H double-stacked cargo for all their plans, and any new freight mainline will have to support that anyways.

> Use above-ground stations.

Comparing with NYC 2nd Avenue Subway is somewhat disingenuous, because this is literally the most expensive subway in the world for construction costs. And there's several reasons for this, but they can largely be summarized as "the MTA is utterly incompetent at any form of cost control and nobody, not even the political overseers, cares two whits about holding them to task for their incompetence."

We actually know how to build subways cheaply. One of the great ways to cheapen their costs is to switch to cut-and-cover for construction, especially for stations which already have to be near the surface. Of course, cut-and-cover is incredibly disruptive, but because the stations still need access to the surface anyways, you end up with much of the disruption for longer periods anyways (because you need to move materials into and out of the underground construction area, so surface access is needed throughout construction).

But this also brings up another point: vertical circulation (moving people vertically). Vertical circulation is extremely challenging to do at high volume; it's one of the biggest bottlenecks in large passenger stations today. The proposed Loop tunnels would have to be underneath existing infrastructure--in NYC, the 7 is at 60 feet below ground--and now you have to somehow get that stuff to the surface expeditiously. A car elevator? That's going to handle about one car a minute. Or you can do ramps, at say 4% grade--that requires 1500 feet of horizontal displacement. Those ramps start eating up a lot of space quickly, and you're supposed to be doing cost stations by eliminating underground infrastructure.

> Use existing electric cars as vehicles instead of specialized train rolling stock.

What a horrible idea. The capacity of a free-flowing highway lane is roughly 2,000 cars per hour, and cars are not particularly easy to get lots of people in and out of them quickly. The typical capacity of a subway line is about 24 trains per hour, with each train able to carry roughly 1,000 people or so. If you plan your subway system well (which means having limited branching, so systems like DC's and NYC's are horribly, horribly screwed), you can push that up to Moscow's 45 trains per hour. Do the math as to how many people that can carry.

Operating costs of subways are likely to be much cheaper than Loop because there's fewer moving parts. And steel-on-steel is more efficient to move on than rubber-on-pavement.

And... I could keep going on tearing down the information here, but it's really low-quality. It's based on hopes and promises of what Musk says Loop could develop, with the convenient fact that it doesn't have any demonstrations of its actual capacity (since COVID-19 means that there's not really any demand for the Las Vegas Convention Center to stress-test it). But the technologies that it is most similar (Personal Rapid Transit) that have been developed and deployed in practice haven't lived up to its hype in the past. Why should we expect this new gadgetbahn to break the model of all the previous ones?