Steam railroad is a term used in the United States to distinguish conventional heavy railroads from street railways, interurban streetcar lines, and other light railways usually dedicated primarily to passenger transport.
Steam railroads were so called because they were typically powered by steam locomotives hauling unpowered passenger or freight cars, as opposed to light railways that were more typically powered by electricity, cable or animal power.
The term steam railroad continued in use in the generic sense long after most steam power disappeared from American railroads by the 1950s, but is now only used to describe tourist railroads that run steam locomotives.
References
Steam railroad Wikipedia(Text) CC BY-SA