Complex sentence: Real estate development was so lucrative for Huntington and SP that they could use the Red Car as a loss leader, however when most of the company's holdings had been developed by 1920 their major income source began to deplete.
Split into simpler sentences: Real estate development was so lucrative for Huntington and SP that they could use the Red Car as a loss leader. However, by 1920, when most of the company's holdings had been developed, their major income source began to deplete.