The Station Beneath The Sea. The Train at the End of the World, #1

When Milo Ren hears a train whistle above the drowned rooftops of his world, he thinks it must be a dream. But the train is real, and it has come for him. The Endline Express appears only at the edge of disaster, carrying passengers between worlds that are moments away from vanishing forever. Its stations open into impossible places, forgotten kingdoms, and dying realms where one choice can mean survival, or the end of everything.
Milo never wanted to be a hero. He is lonely, grieving, and far too young to carry the weight of worlds. But when he is named the train's newest conductor, he must guide a strange group of passengers to their first destination: a breathtaking city beneath the sea, where coral towers glow in the dark, ancient currents whisper through glass tunnels, and something at the heart of the ocean is beginning to fail.
As the underwater world unravels around him, Milo must learn what it truly means to save a place, and whether every world is really given a fair chance. Full of wonder, mystery, adventure, and emotional depth, The Station Beneath the Sea is the thrilling first book in The Train at the End of the World, a fantasy adventure series for readers who love magical journeys, hidden worlds, brave young heroes, and stories where hope arrives at the very last stop.