|First Appearance||Slender Engine-The Eight Trucks|
Slender Engine first appeared in Slender Engine - The Eight Trucks, in which he stalks Percy through Henry's forest as the little tank engine collects the eight missing trucks.
After being asked to be Rosie's valentine, the Slender Engine leaves in his typical manner.
One foggy evening, the Slender Engine stalks Henry near the viaduct, and after catching Henry, it also absorbs the demon, using its power to amplify its attack on the rail network. The Slender Engine continues to make many engines on the Island disappear, causing the remaining engines to group together at the Halloween party in hopes of avoiding him. However, the Slender Engine manages to extinguish the jack-o-lanterns which prohibit him from attacking the engines at the party, and as chaos ensues, he manages to capture all the engines except Thomas and Percy. As he corners the pair, the Halloween Spirit catches the Slender Engine, as he violated the Halloween tenet of never blowing out a jack-o-lantern.
The Slender Engine is a dark spirit who stalks Sodor looking for wayward engines to prey upon. His victims hear him chuffing closer, and seeing him causes one's vision to become stat-icky.
- The Slender Engine is made using Donald or Douglas' body with Neville's tender.
- He is rumored to be the reason many engines (namely Duke, Stepney, and several one-off characters that HIT introduced) disappear from the Island of Sodor.