Ed...
There is virtually nothing of a railfan's interest left on the "Big Island". If you go to the northern tip of the island, to the port of Mahukona, you can see the roadbed of the Hawaii Railway Company, but it's rather boring. (I explored the area, after acquiring a locomotive from the line, #5, and hoping to find some photos, etc. I did find blueprints at the office of the sugar company.)
If you could get to Grove Farm on Oahu, you would see the best stuff there is on the islands. But, my understanding is that visitors are not allowed there.
Don't bother with the "Lahaina, Kaanapali, and Pacific" on Maui. It is nothing more than a trashy tourist attraction, and has no historic merit of any kind.
At Hilo, ask for the harbormaster. He is a railfan, and has explored the island thoroughly. He might have some suggestions for you.
Best regards, Bob Keller