Dating from the Superpower steam engine days. The booster was located on the rear axle of the trailing truck (usually) or the fore axle of the forward truck on the tender. Most booster engines were built by the Franklin Steam Engine Co. Due to the added weight of the larger grate area on the superpower steamers (after about 1925, the Lima A-1 Berkshire being considered the first of that extended family). this weight and the little steam traction engine driving the axle added 10-15,000 lbs additional tractive effort for starting up heavy freights ( they could run up to about 25 mph).