Jack is a friendly and enthusiastic front loader who works for the Sodor Construction Company.
When Jack first arrived on Sodor, the Pack were working very hard to clear ground for railway lines at the quarry. Jack was very excited to be working, but kept jumping into where he didn't belong. Darting too quickly, he took a tumble on the hill and fell into a heap of barrels and crates. After this spectacle, Miss Jenny was not sure if Jack was ready to join the pack, so the friendly little front loader was anxious to prove himself.
His chance came sooner than expected! When a bridge keystone fell loose, Thomas was dangling over the edge with a line of trucks! Jack jumped into action once more (this time for the right reasons!) and used his front loader to hold the bridge into place. This gave Thomas enough time to back away, and Jack reversed before the stone arch could crush him. His front loader was badly twisted, but Miss Jenny was very proud. It was after thus heroic act that Jack has become a permanent member of the team!
Jack was like a toddler at first, who was still learning about the world and how to stay out of trouble. He is brave and will not take any hassle, as seen when he stood up to Max when the latter was bullying him and Alfie.
Jack is painted red with the top half of his cab and his wheels painted cream. He has the number "11" painted in white on his sides. From King of the Railway onwards, the top half of his cab is painted grey, his back wheels are painted grey and his front wheels are painted red.
- Season 1 - Snowdrift (cameo), Scruff the Scrapper (cameo), and Fergus and the Small Engines
- Season 2 - Disappearing Diesel, The Return of Boulder, Duncan's Discovery (cameo), Jack and the Quack, Sodor Day (cameo), Timothy and the Aquarium (cameo), Pile Driver Pedro, Pedro Deep Down (does not speak), Truck Trouble, A Treasury Christmas (cameo), and Ministry (cameo)
- In some promotional sketches of the Pack, Jack is seen with a backhoe arm and bucket. According to an SiF interview with Chris Lloyd, the arm was built, but was too problematic as it was hard to store all of the radio gear inside the model. An interview with Phil Fehrle gives a very different reason: "...if a story required Jack to be digging (using his back-hoe) and interacting with another character working at the same time, his face would be facing away from the action. For story purposes, it was important to feature the faces as much as possible; thus, the backhoe was removed."