I've been nagging Curtis about making a few minor adjustments to some of his early rolling stock into slightly more accurate representations. The first two cars I've been researching are the 30' flat cars used in the 1860s and later.
I have been unable to find much in the way of early UP freight car rosters. I am sure references must exist, but UP since isn't my forte, I don't have them in my library. Thankfully many early UP images are available online in the public domain. The Library of Congress, DeGolyer Library, Online Archive of California and other sites are treasure troves for these old photographs and stereoviews.
Union Pacific has several identifiable types of flat cars in service in early photographs.
400 - 999 (full number range unknown) Series.
These cars share some similarities with the existing UP flat model. The car is probably a 24' with tapered side sills and "captured" end sills. This style of car was also used on the CP and may have come from the same builder. A captured end sill means the end sill fits between the side sills, rather than the full length side sill more commonly used.
Flat #473 is flanked by sister cars #755 and #533 in this photograph. The captured end sill is visible in this view.
The trucks under these cars look very similar to the "Kimball" style truck used under V&T cars, with the addition of heavier spring planks and a leaf spring instead of the coiled spring.