Tuesday afternoon, the Boring Co. gave reporters demonstration rides through the tunnel in modified Tesla Model X SUVs, going between 40 and 50 miles per hour. Engineers have attached deployable alignment wheels to the two front wheels of the Model X. Those alignment wheels stick out to the side of the main wheels and act as a bumper along the track walls inside the tunnel, keeping the Model X on course and preventing the vehicle from running into the side walls of the tunnel.
Also, too, this is a dumb idea even if it works perfectly. It's one of those "I don't even know where to start" things but the capacity will be ridiculously low, speeds never what have been suggested, and like every tunnel/bridge/highway off ramp/etc... bottlenecks at both ends.