Sentence:  To start No. S.002 effectively with a heavy train, one opens the main <WSD>regulator</WSD> sufficiently to move the train off very slowly. An excess of <WSD>regulator</WSD> opening with the by-pass valve open can cause heavy slipping; if the worst happens, the slipping continues a little longer after the <WSD>regulator</WSD> is closed than on an English locomotive. 
Meaning of the word marked with <WSD> tags:  A device that controls the supply of steam to the cylinders of a steam locomotive.