Well my layout has a 2"+ rise in 25 feet up one side and down the other.
so somewhere between 2 and 3% with an O-72 S curve on the uphill leg.
Corners are O-54 with O-72 ends.
My Imperial Big boy has pulled 58 cars on it without seeming to strain at up to 30SMPH. More speed seemed to always pop a coupler. It was smoking and still didn't pop the Z-1000 brick breaker. About 5 or 6 of those cars are lighted, some are PW, but most are modern MTH Railking. Once I got the cars with weak couplers moved to the back or wired shut all was smooth.
I do bunch the coupler slack to get it started. I rarely do this as it fills the mainline and I have to break it down manually. Not enough sidings for all those cars.
