Ryan - The only information I have on the flats and gondola's come from the NCNGRR book by Gerald Best. It is stated that there twenty flat cars transferred from the NCO to the NCNG and numbered in odd numbers 151 through 189. He also states that they were American Car & Foundry built cars although some of them have may not been since the NCO also had their own type of gondola's of which some ended up on the SPNG. The NCNG also bought 21 former F&CC box cars which became their 100 series cars 100 - 140 in even numbers. We have F&CC #507 which is numbered 142. Its possible Herman Darr might have more specific information and I will email him and have him take a look at this thread on the discussion forum.
In regards to tank car 187, Ken Yeo and I used photographs and the F&CC box cars flat plate bolsters as a pattern for the car since there were no surviving examples of those cars that we knew of. John Christensen