In 1969, Union Pacific celebrated the Last Spike centennial with a lot of different publications. The railroad had probably issued reports to shippers before, as they were common in the 1960s, but this one placed a little more emphasis on history than usual.
“The biggest and mightiest steam engine ever built was the famous U.P. ‘Big Boy,'” says the report. “The largest and most powerful diesel locomotives today are the Union Pacific Centennial engines developing 6,600 horsepower.” The former statement was disputable, but the latter was true at the time and may still be true today. While the Centennials are no longer in active service, but so far no Diesels have been made that are more powerful. Of course, shippers were and are more interested in cost and reliability than the power or type of locomotive used to haul their goods.