After you beat the Dark Element,The mayor will expant the railroad.However,the train doesn't have enough Zelmite to go that far.So the mayor has(of corse)you go to Zelmite Mine to get some Zelmite.It has about 35 floors and about 4 dead ends. Plus,every "dead end" you have to fight some gold monsters that have extra strength.

(NOTE1: You get Monica back on the 5th floor)
(NOTE2: I heard that Flotsam somehow comes back)