Yeah, it seems that it would only make sense for very long trips. I imagine you could just use pumps to ensure that there is low pressure even if there is some leakage in the tunnel. Making it underground tunnel would likely help with that as well. It might be more practical for shipping goods than people as well. You could just have this running say between a factory and a port, load up a bunch of goods, drop them off, and go back to pick up more. You could repressurize after the tug leaves too.