There's little need to go 600 in a stretch of just a mile or two. The tunnel has a limited length and is nowhere close to finished. The 600 mph figure was quote

There's little need to go 600 in a stretch of just a mile or two. The tunnel has a limited length and is nowhere close to finished. The 600 mph figure was quoted by hyperloop, which is a proposal by Musk but not an actual business owned by him. Boring Co is building tunnels for electric cars. They will not have a top speed of 600 at any point. Certainly if hyperloop tech gets built at practical scale in the future and if tunnels are needed for parts of it, it would make sense to use boring co for that leg. But the original design of hyperloop was above ground in elevated tubes.