"Understanding your road's traffic is very important" can't be emphasized enough. Bruce mentioned Reading traffic in his PRR area of interest (Columbia, PA, about 20 miles south of Harrisburg on the east side of the Susquehanna river.)
My area of interest is the mainline starting about 30 miles RR west of Columbia. Purchase of several WWII vintage PRR documents on car routing provides some major insights. Yes, for loads, many of us have decided that the distribution of loaded cars can match Bruce's proposed numbers, especially for a mainline functioning as a major US bridge line (as the PRR was between Pittsburgh and east coast destinations, especially during WWII.)
One of the PRR documents I found was specifically on how to route foreign road empties towards their home roads. Keep in mind on the PRR during WWII, there was a tremendous imbalance of loads versus empties. Less than 1% empties EB, 75% empties WB. Ignoring the PRR's large EB coal traffic, 60% of the WB non-hopper cars were empty.
While the 40% of those cars that were loaded might still reflect the national averages, for the empties, there could be significant deviations. Case in point was the NYC - operator of the second largest fleet of box cars. If the yardmaster's followed the PRR guidance, there would be no WB empty NYC cars west of Harrisburg on the PRR mainline. They were sent in a different direction (in fact in central PA, the PRR seems to have enjoyed returning NYC empties to a location on the NYC that was likely very inconvenient for the NYC to deal with ;-) The NYC cars would still be used for loads, but for WB non-hopper trains, the NYC cars should be under-represented.
The other twist was limitations on car clearances, which in the east were often inadequate for many western RR XM's. For the PRR, that meant that the taller XM's would be on trains running between Harrisburg and Chicago, since there were clearance limits along the line towards St. Louis. (The PRR booklet lists every single class of XM for every railroad, with notes on where they could, and could not, be routed.)
While not impacting the average distribution of cars flowing across the railroad, it could have significant impact on the consists of individual merchandise trains, just because of their ultimate destination, For example, all the tall XM's (loads and MTY's) on WB trains were routed towards Chicago (for tall cars destined for the SW US, those cars would use a division in Indiana to get back to the line to St. Louis). XM's on the trains headed directly toward St. Louis would be limited to the shorter XM's. This offers some unique challenges a superintendent may want to impose on their yardmaster when classifying and blocking XM's....