From the Super Mario Wiki
Project K is a railroad from Mario & Luigi: Bowser's Inside Story. It connects three disparate locations in the Mushroom Kingdom: Toad Town Caves, the Tunnel, and Bowser's Castle. Bowser and the Monty Bros. dig the railroad while Bowser is pursuing Fawful to the Toad Town underground. Later, the Fawful Express is fought here.
Bowser can traverse the entirety of the railroad using his spikeball move, though the practicality of doing so is somewhat lacking with the Chakroad being much more convenient.
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