The whole reason the Boring Co.'s tunnels are cheaper is the fact that they don't have to be designed to accommodate specialized electrical equipment or meet the relatively more precise tolerances for rail. Battery-powered rubber-tired vehicles don't need much besides the tunnel bore and a reasonably flat surface to run on. Rail tunnels on the other hand need to be built such that two rails are in perfect alignment with each other, and at a very specific slope based on the train's weight and the power the motors can put out without slipping. Cars can handle the occasional bump in the road and have more traction to stop/start/ascend large grades.
Add to that the long approaches (and therefore land) required for portals and the expense of rail tunneling grows, whereas the Boring Co. can basically drill a long tunnel and dot car elevators anywhere along the route where there's a 20'x20' patch of land and the demand for an access point.
Great information, that's something Ive always wondered about the Boring Company and why he is so focused on people movers as opposed to rail.
I mean, to some extent it's because Musk also has a company making electric vehicles and there are some efficiencies there. I guess in theory someone could hire the Boring Co. to dig tunnels, then have other contractors build the rail infra and vehicles that fit inside the tunnel, but those second two parts will start to run the costs up to the point you might as well have a "traditional" tunneling contractor use a larger TBM to allow for off-the-shelf rolling stock.