The transcontinental railroad promoted the transformation of American industry in at least two ways. It did this by increasing demand and by making it easier to transport goods and commodities from place to place.
One thing the transcontinental railroad did was to increase demand for many kinds of industrial products, particularly steel. The railroad needed steel and other things for its rails and for the rolling stock that went on those rails. By increasing demand for these things, the railroad helped expand US industry dramatically.
The railroad also promoted transformation through its role in transporting goods and commodities. With the railroad, raw materials from one place could be shipped to factories in another. Goods produced in factories in one place could be shipped to consumers all over the country. This transformed American industry, creating a much larger and more interconnected economy. Thus, the transcontinental railroad promoted the transformation of American industry because it demanded large amounts of steel and other things and because it provided transportation that allowed industry to grow.