That old rule seems to apply to everything: there's always that 10%.
I did model the entire system from St. Paul (upper yard that is straight) to Seattle (lower yard that is curved). Since the mainline is 400' long, I figured I could model both yards (cities). Takes a good 1/2 hour to go from one city to the other. Minimum radius is 48" and max grade is about 1/2 percent.
The yards are large being about 22'-30' long holding about 500 cars each with a train of about 35 cars can be in each track. I made all turnouts #10's on the mainline and yards: they are needed in the yards as the cars would probably derail when a switcher is pushing that many cars through the points of the switch.
