>>1219564A lot of the big steam locomotives with drivers in excess of 72 inches did have trouble getting started or pulling at low speeds. The majority of them were fitted with booster trucks. The N&W J Class didn't need them, but types like the SP GS4 had 80 inch drivers and would have struggled without them. Booster trucks were basically a trailing truck fitted with a small but powerful steam engine to help give the locomotive a "shove" they would be disengaged at about 20 miles per hour. For example, the Southern Pacific GS4 had a normal tractive effort of 66,326 ft lbs, but with the booster truck providing an assist, ha a tractive effort of 78,000 ft lbs spfrom a standing start and up to 20 miles per hour. Pic is the cylinder of a booster truck.
