Musk never said you can't put trains in his tunnels. He happens to have a car company to help, but I bet if he had a train company sitting around they would develop something for the tunnels.

Boring company tunnels are wider than London subway tunnels.

Squandering an entire companies potential as a PR move for your other company is hardly big brain executive work though. Film executives do that all the time, and they're some of the dumbest people doing supposedly intellectual work.