The Traincar, also known as just the Train, is a part of the Dense Woods. It is exactly what it sounds like- a lone part of a train, found by traversing the section of Dense Woods B that is only accessible from Hell.
Once inside, the train seemingly begins moving on its own but doesn't go anywhere. A red-eyed person is sitting on the bench inside. To progress, Madotsuki must sit next to the red-eyed person for at least seven seconds. Upon leaving, the sky changes from blue to red and allows Madotsuki to get to Witch's Island by heading left. If Madotsuki walks back into the train, two large-eyed creatures replace the red-eyed person. Sitting between the creatures and bobbing your head again will reset the sky back to blue and will take you back to Dense Woods B.