Complex sentence: They save everyone, but Olber notices that the train driver is still missing and goes back; he finds the driver still alive but unconscious, just as the lava reaches the train and begins melting it.
Split into simpler sentences: They save everyone, but Olber notices that the train driver is still missing and goes back to find him. He finds the driver still alive but unconscious just as the lava reaches the train and begins melting it.