Booster engine
extra cylinders on a steam locomotive, driving the trailing truck or a tender truck, to give more power on starting
en-US/ˈbustɚ ˈɪnd͡ʒən/
en-GB/ˈbuːstə ˈɛnd͡ʒɪn/
extra cylinders on a steam locomotive, driving the trailing truck or a tender truck, to give more power on starting