I'm not sure it's strictly necessary to have the tunnel be low pressure. It's nice for the sake of less resistance, but the speed of sound is actually lower at lower pressures, and so would more readily produce a sonic boom.

It’s worth noting that this isn’t necessarily that big of an issue for China. A lot of their HSR is grade-separated and not infrequently up on viaducts. It’s not a particularly big step in my mind to enclosing that.