The Scammell Pioneer was an off-road design from the late 1920s, built for the Imperial market where made-up roads were scarce. The combination of a suspension with a lot of movement while maintaining traction and a low-revving diesel engine gave it impressive pulling power on rough ground although at low speeds. Its capabilities and performance matched that needed for military vehicles. The [British Army would take the Pioneer for many uses during the war but their first tank transporter based on the Pioneer was a 20-ton capable unit delivered by Scammell in 1932. This led to later 20 and 30-ton tractor/trailer combinations. The trailer was more-or-less fixed to the tractor and not demountable like modern semi-trailer trucks. Hinged ramps were used to get the tank onto the trailer, which if immobilised could be pulled on with the tractor unit's winch. Pioneers fitted with tank transporting trailers had a longer chassis for an extended cab to accommodate the tank crew as passengers, and larger rear wheels.
About 500 tractor-trailer units were provided to the British Army and they were effective recovery vehicles. One problem was found when carrying American tanks on British roads. The higher profile of the US tanks meant that on occasions the vehicles could not pass under bridges. This meant that the trailers were undesirable post war and most were scrapped while the tractors were retained for use with other trailers or sold into civilian use.[citation (source) needed]
add details of machines in preservation, here please.