Gus (Mummy on the Orient Express)

Gus was a computer who represented a force seeking to capture the Foretold and reverse-engineer its abilities. Toward this end, it exposed numerous ships, including the Gloriana and the Valiant Heart, to the artefact that summoned the Foretold and forced their crews to investigate the phenomenon. When the crews inevitably failed to solve the mystery, they would be eliminated to ensure that there were no witnesses.

Personality
Gus was highly intelligent and methodical, as well as a dangerously calculative entity. It also possessed a dark sense of humour, making light of life-or-death situations and openly taunting its pawns. It usually gave off the impression of a high-class individual by speaking with a refined British accent with a cheerful tone - even when murdering its passengers - and using the avatar of a monocle to distinguish its presence.

History
The Orient Express was the last ship to be controlled by Gus for the purpose of analysing the Foretold. Gus arranged for a party of unwitting scientists and historians to be aboard, as well as the Doctor. (TV: Mummy on the Orient Express)

Following the wedding of Amy Pond and Rory Williams, the Eleventh Doctor received a call informing him that an Egyptian goddess was loose on the Orient Express in space. (TV: The Big Bang) The Twelfth Doctor indicated that a phone call to the TARDIS had been one of Gus' methods to lure him aboard the Orient Express, as well as sending the Doctor free tickets. (TV: Mummy on the Orient Express)

Gus assured the compliance of the group with violence against its passengers, blowing the occupants of the kitchen out into space when the Twelfth Doctor refused to stop his telephone call on request.

After the Foretold was identified and disintegrated, Gus explained that survivors were not required and removed the oxygen from the laboratory. The Doctor teleported the living out. When he attempted to find out, through the train's hardware, who had engineered the gathering, Gus blew up the train to prevent it from being revealed. (TV: Mummy on the Orient Express)

Theories
One possible explanation as to who Gus was is that he was created by Missy. Evidence includes:
 * Missy would know what the Doctor would be up to and would also know his phone number to give him the train tickets.
 * Missy would want information on the Foretold for her studies on the dead and dying for her Cybermen army.
 * She has experience with Artificial Intelligence before, like with Seb.
 * The monocle disguising Gus looks suspiciously like 3W's logo.