The Fish Saleswoman is a female character who lives in Papuchia Village in Spirit Tracks. She runs a small business selling fish. Link can buy 20 fish for a total cost of merely 50 Rupees. She can only sell Link the fish if he has obtained the Freight Car.
Link can receive two Force Gems by assisting the Fish Saleswoman.
First, after Link has completed the Fire Temple, the Fish Saleswoman will be in need of Mega Ice. She needs at least 10 units of said ice, causing the hero to haul it all the way from the Snow Realm to the Ocean Realm. If Link is successful in bringing her the necessary items she needs, she'll bestow upon him a Force Gem. This Force Gem restores Spirit Tracks in the Ocean Realm, tracks that even lead to the Pirate Hideout.
The second can be received after Link transports at least 15 fish to a woman in Hyrule Castle. She'll be grateful that Link made the trip, and reward him with a Force Gem. This Force Gem also restores tracks in the Ocean Realm.