Eric, a young boy, is excited about his birthday after reading a book and believes that a train will come for him, much to his teenage sister Jill's disbelief. A few hours later, the train station's control towerwakes up and in the roundhouse, Tillie, a young little blue switcher engine, along with her best bird friend, Chip, wakes up four other trains: Georgia, a kind all-purpose engine; Farnsworth, a stuck-up, shiny, new passenger engine; Jebediah, a worn-out, rusty, old engine; and Pete, a gruff, burly, big, strong freight engine. The tower assigns each engine except Tillie a train to go over a nearby mountain, but angrily declines Tillie's offer to allow Jebediah to rest and take his milk train. Georgia, who senses something wrong with her mechanics, is assigned to pull the birthday train. A clown named Rollo leads the toys and friends into the train, including Jeepers, a monkey; Stretch, a basketball player; Missy, a ballerina; Handy Pandy, a panda; Perky, an elephant; and Grumpella, a pessimistic stuffed bird. During her journey, Georgia breaks down; a medical train orders Georgia to return to the roundhouse for repairs, telling the toys that they will have to flag down another engine to complete their journey on time. Farnsworth and Pete turn blind eyes to the toys' plea, and Jebediah, though willing, says he is too old and weak to do the job. The tower again rejects Tillie's offer to helm the birthday train, so she and Chip sneak past the tower and pulls the birthday train up a mountain. Despite being ridiculed by the animals along the way, Tillie manages to reach the summit after crossing a bridge that collapses but loses the last train car, which snaps off and falls into the river along the way. When Tillie sees a scary cave, she tries to go through it. However, the cave says that she is too little. Unfortunately, an avalanche knocks Tillie out cold. Eric is woken up by a thunderstorm and is worried about Tillie and the birthday train. After Tillie regains consciousness, she pulls the train out of the snow using her cowcatcher as a snowplow and down the mountain before they reach the town, much to Eric's delight and Jill's amazement. Although very tired from her long and adventurous journey over the mountain, Tillie is very proud and happy that she could and tells her best friend Chip that it was worth it.
