What caused the gross domestic product of the united states to quadruple between 1860 and 1890?

The correct answer is the new industrial economy and railroads

Industrialization and construction of railroads transformed the American economy. As industrialization took hold, the US gross domestic product, that is the output of all goods and services produced annually, quadrupled- from $9 billion in 1860 to $37 billion in 1890.

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