Complex sentence: Upon starting, semi-compound working allowed boiler steam to be admitted directly to the low-pressure cylinders through a reducing valve, to obtain greater tractive effort and to avoid starting trouble due to the high-pressure crank being at or near dead centre.
Split into simpler sentences: Upon starting, semi-compound working allowed boiler steam to be admitted directly to the low-pressure cylinders through a reducing valve. This obtained greater tractive effort and avoided starting trouble due to the high-pressure crank being at or near dead centre.