According to the American Short Line and Regional Railroad Association, short line railroads fall into two categories:
A. Local railroads are line-haul railroads below the regional criteria plus switching and terminal railroads.
B. Switching and terminal railroads are railroads that are either jointly owned by two railroads for the purpose of transferring cars between railroads or operate solely within a facility or group of facilities.
More information at the ASLRRA website:
For a definition of "shortline" railroads:
The ASLRRA home page:
The Wikipedia definition: