My train building has skylights. I mostly work on trains during the day so I don't worry too much about artificial light. Right now I'm using the lights on the ceiling fans but I do plan to put in track lighting eventually.
Since your trains are in a separate, above-ground building, you might want to think about skylights. They give a bright, diffused, natural light that's great for working on the layout. Whether they are worthwhile depends on how much sunlight you get and how much time you spend in the train room during the day vs. at night.
