Which circumstance was most directly responsible for the rapid growth of the railroad industry?

Inventors developed air brakes and more efficient train-car connectors

Builders agreed on a universal width for train tracks

Railroad owners sold stock to investors to finance the construction of railroads

Pullman began manufacturing cars for overnight travel

Option C - Railroad owners sold stock to investors to finance the construction of railroads

This was the most important reason that was directly responsible for the rapid growth of the railroad industry because constructing railways was a very expensive capital investment business. Hence railroad owners sold stock to investors to finance the construction of railroads.