Thomas and the Evil Diesel is a ninth episode of the first season of Thomas' Adventures Chronicles.
After Diesel made another trouble to our heroes, Rainbow Dash thinks that she just had enough with him and decided to make Diesel into a reformed engine. But Thomas told Rainbow Dash that Diesel is not always been bad (which puzzles Twilight, Spike, Applejack, Rainbow Dash, Princess Irene and Curide about it). Thomas told them that Diesel have rescued him once on his second vist to Sodor when he slips on the Oily rails while taking passenges home from market day. Thomas diecided to tell them a story about what he meant about Diesel.
When Percy has to go to the Works for repairs, Diesel returns to Sodor and, as expected, causes trouble for the engines by destroying the oldest truck in Ffarquhar Yards. But two days later, Thomas has an accident when Daisy drips her oil on the track and Clarabel's back wheels come off the rails at the special points at Dryaw, so Diesel comes to the rescue. It seems that even Diesel has some good in him somewhere.
(End of Story)
After Thomas had finish his story, Twilight had to admit that Diesel is not always been so bad and Rainbow Dash agreed. Thomas then also said that someday Diesel might become a reformed engine and turn to our sides.