Here's a perspective from an N scaler who's modeling a bridge line (D&RGW's Soldier Summit, in 1965). The point of my layout is to get 8 trains (2 passenger, 6 freight) up the grade to Summit, in the face of the uncomfortable fact that there are an additional 8 trains (again, 2 passenger, 6 freight) trying to get down the grade... not to mention the returning helper units, which also need to get down. My layout has 2 precent grades (slightly less than the prototype). I've found that I can operate trains with 24 cars without risk of derailment. So, for the bridge line staging alone, I need, at the minimum, 288 freight cars.
To model the mid-1960s-era Summit traffic realistically, though, I need more than 288. During the mid-1960s, a typical Rio Grande manifest freight included something like 20 percent SP equipment and an additional 10 percent each (roughly) from the Grande and its other interchange partners (WP, CB&Q, CRI&P, and MP). The additional 30 percent could be from -- well, anywhere. Every one of these trains was unique and surprising -- you just never knew what you'd see in the consist! To come up with sufficient variety to compose realistic-looking trains, with a surprise or two in each consist, I find that I need at least another 100 cars.
There is some switching on my planned layout, involving coal hoppers. For the Castle Gate coal mine, I need about 35 -50 hoppers to realistically simulate the movement of empties through the mine to the yard for loaded cars, waiting for a local to pick them up. To model Helper's Upper Yard realistically, I need another 75-100.
Bryan Pfaffenberger
Charlottesville, VA