There is unused text in the Japanese version hinting at an unused train system. According to the text, Mario could purchase tickets, get them stamped, and travel to different parts of Isle Delfino, possibly as means of accessing the different levels in the game. Since Pinna Park is one of the listed locations, this implies that either there was a bridge to the island at one point, the train was a subway, or Pinna Park wasn't always located on the tail island of Isle Delfino. Note that the text also hints at three possibly unused areas, named "Battleship Island", "In front of Flame Temple", and "Lighthouse Island".
Contributed by game4brains