From a purely engineering standpoint, yes, it could be done, that's not the problem. The problem is you're either going to balloon the cost to a level where aviation fuel would need to hit $1000/gal before taking the train is more economical, or you're going to be adding so many curves that you'll never hit bullet train speeds.

tunnels are absurdly expensive to maintain , the turnpike keeps getting rid of them due to the cost associated with them