Answer:
The building of the cross country railway was the major factor.
Step-by-step explanation:
The development of a nation-wide railway network, that could connect the east coast with the wild west is the major factor that helped to industrialization in the US.
Before the development of the railway network, large societies in the West and South of the country were highly disconnected with the more developed north-west.
However, railroads made it easier and cheaper to travel and transport goods, giving rise to a boom in local manufacturing and even export.