What do you do after you beat the train on Wild West Island?

Only 2 of the Tonics are useful. The Concentration Carbonate may help on the train robbery and you need the Transparency Tonic to slip into the gang's hideout later in the game.

Once you have used the nugget to buy the Tonics, you should fix the Clock Tower in Diamond Plains to bring the train into the station. Once you defeat the train robbery by the gang, you will receive half of the hideout map. The other half comes from the big-head kid, who can be cured by recovering the Blue Tulip from the mine at the upper right in Rock Ridge.