What were most railroads built on?

This is difficult to answer because depending upon the area and the railway, and era, the answer is different. The early railroads used wooden rails with steel straps on top, which was followed by iron rails. Initially, these were built on stone "sleepers", but later were built on wooden sleepers or ties. In the Southern US, many railroads were built on "stilts", essentially bridges because of the wetlands/swamps they travelled through.