Be sure to go to Grove Farm. If it isn't an operating day ask around and the steam guy may be there anyway working on some of the equipment. I was given his phone number and he agreed to meet me on a Tuesday and showed me their railroad as well as let me run their diesel. He also took me to the old engine house up above the current railroad (about a mile away) and opened it up to show me the other three steamers.
I spent an entire afternoon with him (can't remember his name, will try to find it later-a great guy!) and never got to the other plantation museum.
There's also concrete bridges with track in them north of Lihue that can be visited.