Secret of the Green Engine is a Step into Reading book based on Blue Mountain Mystery.
Thomas is going to the Blue Mountain Quarry and is happy to help there. He sees a green engine that he does not know and follows him. Both engines speed up. Soon they come to a broken bridge. Thomas tells the green engine to stop and he stops just in time. The green engine says his name is Luke and that he has to hide or he will be sent away because, a long time ago, he bumped a yellow engine into the sea by mistake. He also says that if the yellow engine is found, he will not have to hide anymore, so Thomas wants to find the yellow engine. He searches the countryside, Tidmouth Sheds, the Docks, and the Sodor Steamworks. There, Victor says he was the yellow engine. After Cranky had lifted Victor out of the water, he was given a new coat of paint. Now Luke does not have to hide anymore and he starts working at the quarry. Thomas and Victor have a new friend.
- Gordon (does not speak)
- Percy (does not speak)
- Cranky (does not speak)
- Skarloey (cameo)
- Rheneas (cameo)
- Sir Handel (cameo)
- Rusty (cameo)
- The ladder going up to Cranky's cab is missing.
- In one illustration, Rusty's face is rectangular.