My husband is an obsessive railroad fan and my Daughter likes them too. For her second birthday we did a train theme. Not Thomas, just generic trains. We decorated in pink and blue (the colors of Osh Kosh's railroad overalls) and had lots of train touches. There was a train pinata and we put up Katie's Wooden Railroad for the kids to play with. I had a little outfit made for her with a pink 'girly' train and most of the guest kids wore the Osh Kosh railroad overalls all on their own. The kids got train whistles and bandanas in the appropriate colors as favors, and you can see the cake that we had made below:
Obviously an odd theme for a girl, but she is 6 and still loves to see the pictures.
I am really glad that we did something generic and not specific like Thomas. If I were you, I'd go for the Luau! There are so many cute things you can do! Oriental Trading Company has pages and pages of stuff!