From Bulbapedia, the community-driven Pokémon encyclopedia.
Transit Town (Japanese: ボウゼタウン Bōze Town) is an anime-exclusive location in Johto, specifically a port town on the northeastern tip of Red Rock Isle. It has one ferry that goes to Olivine City and another that goes to Silver Rock Isle.
After visiting Diglett Village on Red Rock Isle, Ash and his friends decided to take the ferry to Silver Rock Isle. They visited the town off-screen at the end of Plant It Now... Diglett Later to take the ferry, but returned in Throwing in the Noctowl to take the ferry to Olivine City. They missed it, however, but met Wings Alexander, a resident of Transit Town and a pilot who was also a skilled Pokémon Trainer in his youth. Wings eventually took them to Olivine City with his plane.
Pokémon seen in Transit Town