He was worth a million of dollars because he was very big in the railroad industry. He also had most of the big railroads all across the country that he used.
Vanderbilt used these railroads to make money by transporting peoples goods that they needed to get in some places. They paid him to transport their things to like another state. He had a lot of competition for the railroads and one of his main competitors was John D. Rockefeller. They both had a lot to transport, but for a while Vanderbilt had the lead, then later on Rockefeller took it from him and kept it for a long time. Not only did he own the railroads he built many miles of railways.