Sentence:  ...viz. the railway : an invention which is so thoroughly English that, even supposing it likely to have occurred to any slow Prussian <WSD>brainlet</WSD>, it would still have been impossible to have found any other nation (excepting perhaps America) with spirit or capital sufficient to have afforded the plan a trial. 
Meaning of the word marked with <WSD> tags:  A dolt; a fool; someone having a small brain.