Last year I summarized the history of ALCO and used S scale models to represent many of the important ALCO locomotives. The video below is a follow-up: a history of EMC/EMD also using a variety of S scale EMD locomotives wherever possible. It was EMD that made the greatest contributions to the switch from steam to diesel power in America, and this video covers that switch and the decisions behind the designs of some of the most widely recognized faces in American railroading.
https://www.youtube.com/watch?v=G30hgCwL4x0
Terry O'Kelly