Newman Atkinson says:

I don’t know the gradient of some of the climbs in the Rockies but in model railroading the shallower the climb the better. Model engines do not have the power and weight ratio that the proto engines do. A 2 percent is probably the highest grade you should pursue. I have seen folks build grades of 4 % or more and a very short train might go up it but nothing that you would expect to look like a decent train.
In Sales of trains, the MFG’s promote the high grade to climb over another track and then immediately right back down. Lionel was a big promoter of that for years. Now tell me just where you might find a track that would climb like a car overpass and right back down. I know of none. The tracks would stay level across a ravine or track below it. A track would not go up and then immediately back down. But I have see many who have done it and they never work. You want to keep the grade as slow as possible. If you don’t you will not be able to climb with believe-able length of trains. in HO Scale a 2 % grade would be a 2 inch climb in 100 inches in travel. It doesn’t matter if you scale it as inches or some other length measure, It will be 2 of what ever measurement to 100 of the same. I deal for models would be 1 inch for 100 inches of travel or 1% We in model railroading have only so much space in our basements or garages but if at all possible you will want the least climb as possible. Just the climb will be well represented just because it is there. from Newman Atkinson