Foster Yeoman issued a tender document which requested 95% reliability. General Motors' bid was ultimately successful, in particular because their proposed design, derived from the EMD SD40-2, was equipped with the well-proven Super Series creep control, which allows superior traction at very low speeds. This, it was found, would enable a single locomotive to haul Foster Yeoman's 4,300 tonne stone trains, whilst two Class 56 or Class 58 engines would be needed to move the same load. This enabled Foster Yeoman to reduce its requirement from the original six locomotives to four.