Is there a definitive source of knowledge regarding passenger car trucks and why so much variation? Thank you
Wow, that's a pretty tall order. As far as the variation you could say the same thing about automobiles over the past, say, hundred years. They developed over time, based on needs and technological advancements. Weights of the cars, speeds of the trains being paramount.
In some instances a particular railroad may have favored one type over another. The PRR, of course, could be an entire book unto itself with their one-of-a-kind types and "home-brewed" varieties. **
I have several books on the subject. A Car Builder's Cyclopedia from the era you are looking for would be a great source.
Additionally, the two-volume set titled The American Railroad Passenger Car by John H. White, Jr. is good also the two-volume Illustrated Treasury of Pullman Standard Passenger Cars Since 1945 by O. M. Kerr or any one of the Official Pullman-Standard Library volumes by W. David Randall which is divided into books focusing on one or more groups of specific railroads. He also wrote a series on Budd and A. C. F. cars.
I seem to recall Trains and/or MR Magazine having an article on passenger trucks in a .pdf. Right now I can only find the one on freight trucks. I'll keep looking.
Here's a quick overview from RPCA, the private car folks:
http://www.rpca.com/pdfs/passengercartrucks.pdf
It can be overwhelming, to be sure, but like any kind of research you have to break it down into smaller chunks and look at what specifically pertains to the cars in question.
** http://varnish.pennsyrr.com/PRRTrucks/DGS_PRR_PTrucks.htm
[edit] It was the Feb. 2005 MR that had a Modeler's Guide To Lightweight Passenger Trucks.
I look forward to seeing what others contribute as well.
Have Fun, Ed