Back to the sci-fi universe. This story, like most of the side stories/spin-offs take place prior to the appearance of the second generation students (i.e. Alice and Cody).
Jade builds a time machine. Christy is studying history from the early 1900, specifically an event which took place on the newly built railway lines. Ray is practicing his English. Syrus is... being Syrus. Bam. Jade somehow accidentally sends them back in time, conveniently to the period Christy is studying. They discover they're on the train that Christy knows a lot about. She's scared. There's an event about to happen.
Jade needs to buy some time and fix the time machine in order to send them back to the present. Christy warns them all, in her studies, a masked man in a grotesque monster mask boards the train and murders every single person on board -- and now they are on board, they're fresh targets! The students/spies use their skills together to evade the serial killer and track down his true identity before many people are hurt, eventually capturing and unmasking him (and giving him to proper authorities). Just in time, Jade fixes the time machine and they all go back to the present. Christy's history books has changed. What was then a tragedy is a mysteriously heroic event, as what would have been a terrible event was stopped by four mysterious, disappearing teenagers.