The mysterious Gus, voiced by John Sessions, was never actually revealed in this episode, though he master-minded the whole affair. He set the mummy loose upon the passengers and crew and he filled the train with the best scientific minds to study the creature. The Doctor may have solved the mystery but he still thwarted Gus's plans by saving the people on the Orient Express from the rigged explosion. By the end of Mummy On The Orient Express we knew no more about Gus then when the episode started, suggesting that there is more story still to come. One of the burning questions is the identify of this mysterious villain. Does he have a connection to the Doctor? Is he part of the Promised Land series arc? Is he - as is the big rumour for series eight - a rebel time lord escaped from time-locked Gallifrey ? Is he the Master? Either Gus is a one off villain shrouded in mystery or this is the start of something greater. Our money is on a fellow time-lord out for revenge. The Meddling Monk perhaps?