Why not, Tidus says. Yes, why not see how long it takes for you to literally disappear into nothingness in the mdidle of a train. Why not.

Though it's not that thought that Inigo lingers on as long. Not while the other continues with those words he says later on. Words that makes the look in Inigo's eyes even a bit sadder.

"Why should you know?" Inigo asks, his voice a tad softer.

It's definitely meant as an open question. Why does Tidus think he should know that? When there's no way that this is what Tidus wants to be, even if it's his reality.