Here are pertinent excerpts from a 1917/1924 textbook titled Design of Railway Location that some modelers may find useful.
“The outer rail of a railroad track on a curve is placed at a higher elevation than the inner rail in order that the resultant of the centrifugal force and the weight of the train may be as nearly as practicable normal to the plane of the track, and consequently make the wheel pressures on the two rails equal…If all trains passing around a given curve were operated at the same speed, the proper amount of superelevation could be calculated according to the principles of mechanics. The truth is, however, that the trains are operated at widely different speeds…If the freight traffic over a line predominates and the passenger traffic is very light, then the speeds of the former should govern and the passenger trains should be required to slacken to a safe speed for curves…Usually a compromise speed is adopted for the sharper curves…A limit is usually set empirically to the amount of superelevation that may be used, different railroads having different standards in this respect, 6 to 8 inches being common practice…On track used exclusively for freight, the superelevation should be calculated for speeds not exceeding 15 miles per hour, for it is less expensive to decrease speed on the trains running down grades than it is to overcome the additional resistance due to unbalanced superelevation. An instance is recorded where two engines hauling 3500 tons of coal on an 8 degree, 30 second curve with a central angle of 185 degrees habitually stalled when the superelevation was 5.5 inches, but when it was reduced to 3 inches no further difficulty was experienced.”
The above gives guidance was the thinking when railroads were at their height. Besides giving some rule-of-thumb on the amounts of superelevation used, my principal lesson is that superelevation should be less on rising/falling grades compared to level grades because trains will usually be moving slow