Final answer:
Andrew Carnegie turned to J. P. Morgan to sell steel when railroads no longer built new lines.
Step-by-step explanation:
When railroads no longer built new lines, Andrew Carnegie turned to J. P. Morgan to sell steel. Carnegie sold his Carnegie Steel Factory to J. P. Morgan, who created the United States Steel Corporation, the largest corporation in the world at the time. This allowed Carnegie to continue selling steel and expand his business.