The Automatic Train is a functioning military vehicle used to travel to D6. The vehicle gets its pseudo-name from its first encounter in which it arrives to the Rangers, apparently after they triggered a boarding sensor.
Easily fitting into the category of "fast silver trains", the automatic train is an unmodified passenger train that is still in fine condition, despite having had 20+ years of no maintenance. It is unknown how many of them there are as there are several of these trains located in D6's main hub, and presumably several more in the surrounding tunnels. In Metro: Last Light, the Rangers use these trains for quick transport across the metro using the secret Metro-2 lines the trains operate on (a service station on that line leads to the Botanical Garden).