gauge is likely easy to change. Not cheap, but Amtrak demands expensive inspections and refurbishment to run, so the cost of changing the gauge is likely fairly small compared to the other costs.
Off topic, but some trains can even change gauge while in motion: https://www.reddit.com/r/trains/comments/kq6eds/this_is_how_...
Off topic, but some trains can even change gauge while in motion: https://www.reddit.com/r/trains/comments/kq6eds/this_is_how_...