Forget faster than driving, it's not even faster than the current ONTC bus. This is why the business case is looking at an overnight service.
From page 27 of the business case:
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During the daytime, the train would provide little improvement over an existing express bus. But for overnight trips speed is less important since you'd want to sleep for 8 hours anyway. Instead, comfort becomes more important, which is somewhere the train has a big advantage since its larger size makes it practical to provide much more space per passenger.
Ideally, they should be offering some sleeping accommodations such as beds or at least lie-flat seats, which would attract some customers who would be dissuaded by spending the night in a chair, and can afford a higher ticket price. But I'm not sure how much sleeping accommodations they are actually planning, if any, given that the fleet they're purchasing only has 3 single-deck coaches per train.