Jack is a friendly and enthusiastic front loader.
It is unclear what type of vehicle Jack is based on, but between the sixth and twelfth season of the TV series 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.