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

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

Constructing railways was and still is a very expensive capital intensive business. Not only did it require large gangs of workers but the land had to be bought and the tracks themselves also had to be laid. To do that the rail companies sold shares to investors and raised the money.

the enslaved people onboard suffered from illness with some frequency

the anwser is a) to prevent military conflict