The ads for car builders' supplies are as informative as the articles. An occasional publication by the Railroad Gazette until the turn of the century. Eventually, as intended, it became the definitive compendium of railroad carbuilding terminology and practices. In 1892 William Voss wrote a textbook giving detailed technical descriptions should be fitted, and discussed new innovations and why they were being adopted. These are better sources of information in earlier years because the Southern Pacific reported on a single report for all lines owned or leased.
Court cases are interesting for both the testimony recorded as well as lists of assets.
More valuable for the names of the organizers than for car information. Like any business railroads kept records of their activities. When these records are available they will be among the richest and most reliable source of records relating to any specific car. Remember that each department had separate needs and therefore kept different kinds of records. Indexing will be by the primary subject and subject to the whim and knowledge of the indexer.