Mike, I mentioned this in a response on the MRR forum, but I will expand on it a little.
There are a few things that will determine your choice of fright cars for a consist.
1. Date. Do you have a cut off date, say 1975? Anything newer than that would be not yet made so off bounds for purchase.
2. Industries. Does your model serve any specific industries that might have an influence over the rolling stock? For example private label freight cars for a specific industry.
3. History. What are the predecessor railroads that would still have freight cars painted and lettered that are part of the BN heritage. The closer you are to the formation of BN (1970?) the more of these there would be. CB&Q, Northern Pacific, Great Northern etc. Some of these would likely be old and well weathered in your era.
4. Interchange. Are there any notable roads that interchange with the BN in the area you are modelling. Freight cars from these roads and virtually any other could and would show up in a consist.
The bottom line is that it is very unlikely that you would see a consist made up of entirely BN cars. There would likely be a mixture, some nice new BN green and plently of old weathered GN, Q etc.
Setting yourself a nominal date range for the cars can help in avoiding purchases that in the end don't fit. You don't have to be a complete slave to this, but it can help in assembling a more realistic roster.