originally, rail road wheels were made of cast iron (main spotting feature is the radial ribs on the back for cooling the wheels during braking) Now, wheels are forged from steel
The wheel was invented because people realized it is easier to move heavy objects when something rounded (like logs) were put under it and they rolled it to its destination.
The rail roads in the United States were built by Chinese and Asian slaves during the mid to late 1800's. They were not officaly slaves. What companies did was pay them (for example) $10 a day, but...