The Tower of Nero

Apollo, the god of the sun, has become human teenager Lester Papadopoulos as punishment by his father, arch-god Zeus. Apollo and his demigod master Meg McCaffrey have been traveling across the United States to rescue sources of prophecy or Oracles, which have been seized by a trio of resurrected Ancient Roman emperors, known as the Triumvirate. In the fourth book of the Trials of Apollo series, Apollo and his friends managed to defeat Commodus and Caligula, two members of the Triumvirate. On their final quest, Apollo and Meg take a train to Manhattan, where Nero, the surviving and most dangerous Roman emperor lives. Throughout the series, Apollo is given prophecies to guide him on his various quests; at this point, he has received only the first part of the prophecy for his final mission. The prophecy sends him to seek out Nero, the most powerful of the triumvirate, in New York.

In Union Station at Washington, DC, Apollo notices an amphisbaena, a snake with a head on either end, board the quiet car dressed as a human. The other travelers cannot notice the snake as the mist, a bit of magic, hides supernatural reality from them.