Tesla's design [for Wardenclyffe] used a concept of a charged conductive upper layer in the atmosphere, a theory dating back to an 1872 idea for a proposed wireless power system by Mahlon Loomis. Tesla not only believed that he could use this layer as his return path in his electrical conduction system, but that the power flowing through it would make it glow, providing night time lighting for cities and shipping lanes.
It's a very Victorian / Steampunk idea that is also kind of horrifying. It's working off theories of what electricity "is" that we now know aren't accurate, but if you try to scale them to actually working every building and tree and person is now a lightning rod.
Part of the problem is to meet their quoted throughput of passengers they would need to fully load/unload each vehicle in ~30 seconds. 4 adults in, 4 out, with luggage, with no delays or struggling. That's... not very feasible for a commercial passenger car. They're not designed for quick loading and unloading.
The tunnels are a single lane without a service tunnel, which the Victorians used in the 1800s for their subways. Because if a single car has mechanical issues the entire service has to stop and empty to clear it. They're electric, so there are less mechanical systems, but they are still putting a significant amount of wear and tear on tires/axels/steering systems, all the mechanical systems they still have. Even without meeting the their goal throughput, they're putting orders of magnitude more use on each vehicle, which are consumer cars. They're meant to spend most of their lives parked.
If they made a "Tesla train/trolly" where the engine car was pulling a simple enclosed cart with seats it would significantly improve their throughput and loading times, and require less maintenance per passenger. But at that point you've just invented a train that uses significantly less efficient rubber tires on asphalt.