How did andrew carnegie make his fortune?

Andrew Carnegie emigrated to the Us at the age of 13 and worked his way up from working in a factory to a superintendant on the railroads. 
he made his first money by investing in oil, and started a steel company. His business method was to own the means of production, ie he controlled the process from start to finish and wasn't paying someone else for raw materials or to make stuff for him. However, he didn't treat his workers that well (lowering wages etc). 
In 1901 he sold his business and decided to use his wealth to help others. 
So I guess the short version is; he had a good eye for an opportunity/ a good investment, was lucky to have been in the right place at the right time (getting in on the start of railroads, then oil, then steel production), knew his industry well, and could be ruthless in business.