I think we should work together with the diesels after this!
Steamies and Diesels is the fifteenth episode of the thirteenth season.
One afternoon, Toby passes the quarry pulling a train of tankers full of diesel fuel, when Bill tells him to turn around and let the diesels wait. Since Diesel 10 called Toby an "out-of-date little engine", he agrees and leaves the tankers at the quarry.
The next morning, Toby goes to fill up with coal, only to find all the bunkers empty as Diesel 10 was supposed to bring a fresh supply. Mr. Hatt arrives, saying that he is arranging for some lorries to deliver the engines' fuel, but lots of passengers will be late as a result. Toby feels ashamed, and Thomas tells him that they should work together with the diesels from now on.