These two articles might be handy for figuring what cars make up a consist and the how old or new the prime mover might be that powers that consist.
Here is a link to a pdf that lists the type and percentage of rail cars in the US fleet. This gives you an idea of what cars are most common in the modern consist. It also has a graph that lists how many prime movers were manufactured and when, giving a idea of how old the average fleets are. Plus a whole bunch of other information.
https://www.usitc.gov/publications/332/ITS-08.pdf
Two more links that cover the same subject but might be more to the point.
https://www.statista.com/statistics/571 ... s-by-type/
https://www.statista.com/statistics/185 ... ince-1990/
Here is a link to Model Railroader magazine that covers what type of cars can be placed together or should be separated because of Hazmat.
http://cs.trains.com/mrr/f/13/p/184080/2014124.aspx