Train over the Abyss

This is a story about the Dragon Quest, except that the dragon turns into a demon from another space, and the hero's sword turns into a train. The whirlpool in another space that appeared out of thin air attracted demons and destroyed the splendid civilization on earth. The surviving humans used the energy generated by the "Eternal Motion Silence Train" to build a city in the sky thousands of meters away from the vortex. After fighting against demons for more than a hundred years, observers in the palace detected that the vortex energy was becoming disordered, and it seemed that the time was ripe to completely eliminate the demons. The current king summoned brave men to launch a star expedition, vowing to regain the autonomy of the planet! However, the demon hiding in the whirlpool has been waiting for a long time.