That really is David Duchovny jumping onto the train at the climax of this episode. The stunt took 6 weeks to set up.
Online forums frequently talked about Mulder's habit of always losing his gun. So the loss of his gun was deliberately written into this episode.
First appearance of Agent Pendrell.
An 11 year old boy played the alien in the autopsy scene.
It won 2 Emmy Awards for Outstanding Sound Editing for a Series and Outstanding Sound Mixing for a Drama Series.