The monorail transport was some kind of light-to-medium rail system that transported materials across Cybertron.
Dreamwave Generation One comics
Beast Machines cartoon
While Tank Drones were doing it to target, Optimus fell into a monorail, and saw Rattrap hidden in it. The Tank Drones continued shooting at the monorail until it exploding. From there, they jumped off the monorail, he exploded and they fled from there, but the Tank Drones would not stop chasing him.