Jack is a friendly and enthusiastic front loader, who works for the Sodor Construction Company.
When Jack first arrived on Sodor, he worked with the Pack to clear room for railway lines. Jack did well with Alfie - that is, until he tried to take some stone up a ledge but lost balance and tipped over. Jack learned his lesson, and redeemed himself by holding up a collapsing bridge until Thomas had passed over, at the cost of damaging his arms. Jack has since become part of the Pack.
When Jack was being careless at the warehouse, he knocked over some slate, but he didn't want to be sent back to the yards and left the mess not knowing that Ned was blamed for it and was being sent back to the yards, until he heard from Alfie and admitted that he broke the slate and was sent back to the yards.
He, along with the rest of the Pack, helped with the restoration of Great Waterton and worked with Alfie to construct a bandstand there. Jack also helped with the restoration of Ulfstead Castle and later saved Stephen and Thomas after the tracks they were on collapsed.
In Sodor's Legend of the Lost Treasure, He also helped build the Harwick Branch Line, along with Alfie, Oliver, Max and Monty, and later attended the branch line's opening ceremony.
In the twentieth season, he, Alfie, Oliver, Max and Monty were working on Thomas' Branch Line and he, Oliver and Alfie helped cleared the branch line in Rail Under Road Bridge and Signalbox after Max and Monty dumped their loads on the line.
Jack is brave and will not take any hassle, as seen whenever he stands up to Max and Monty when they bully him and Alfie. He is also kind and caring.
It is unclear what type of vehicle Jack is based on, but between the sixth and twelfth season he seemed to resemble a Nuffield tractor with an added front loader.
Since King of the Railway, Jack's basis has closely resembled an International 454 tractor. Because of this, his face shape has changed dramatically from round to rectangular.
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.