The Train Station unlocks after you rescue Conductor Kappy. Once unlocked, you can complete train orders from other cities.
To complete a train order, tap on a train car’s sign to see the number of items needed. If you have the required amount, tapping the Load button fills the train car. If you do not have enough, tapping the Load button allows you to purchase the remaining items needed using gems.
Once you’ve filled all of the train cars on the train, the train departs for its destination and returns with gifts from the cities you just helped.