Included are concise histories for the largest transit systems, including the historic companies which had previously operated the public transportation systems in each city, and years of operation of each type of public transportation vehicle.
Streetcar companies are mentioned in the Moody and the Poor investment manuals, the McGraw Electric Railway Directories, the McGraw publication Street Railway Journal and its successor Electric Railway Journal, and "The Trolley And Interurban Directory", by Joseph Gross. But it is possible, that some streetcar systems failed to be completed, and were never actually operated.