I did some testing last night of how much my new Atlas engine (N-scale) could pull going up an incline. While it was a very steep incline (4.27%!!) over about 58.5 inches, the engine could pull only 4-5 cars depending on the length of the cars up the incline.
This was done fairly via a dirty method using my KATO Track and the standard piers in the pier set. While I don’t plan on using this huge of an incline on my layout for N, would another brand have more pulling power an incline say a KATO. Also would a 6-axle have more power due to more traction with the rail?
Your results are not out of the norm. On a 4+% grade, very few locomotives will pull more than 25% of what they will pull on level track, and often even a lower percentage, especially in N scale. Locomotives with traction tires or heavier weight will almost always do better than lighter or all metal wheel treads. Whether 6 axle power will do better depends on whether all axles are powered, and the weight of the locomotive.
The more free-rolling the cars, the more cars the engine will pull on both level track and grades. But the percentage reduction in number of cars pulled up a given grade will actually be higher for free-rolling cars.
Lighter cars also increase the number of cars pulled on grades, but have little to no impact on level pulling power.
General rule of thumb is 2% grade will cut train length by 50%, 4% grade cuts train length by 75% or more. Curves, depending on sharpness, generally add 1% to 1.5% to the effective grade. For these reasons, a 4% grade is going to limit you to short trains or helper operations.
I ran some tests on a new Atlas RS11s, up a 2-percent grade on a 24" radius helix. It managed to lug 13 cars up the grade, all with plastic wheels (Atlas, MicroTrains and Intermountain cars).
The pulling power is more dictated by weight than the number of drive wheels in N scale. For example, Kato E8s pulled 15 or so cars up the same grade without any effort at all. I haven’t added cars to the max for the E8s, but I would expect them to haul somewhere between 20-25 before they start to slip. But those are noticeably heavier than the RS11s. Life Like’s FA/FB 1&2s will pretty much pull the refrigerator, but they are very heavy for 4 wheel truck units. Mine are in pieces getting decoders installed, so they weren’t part of that test. If I remember the test results, Intermountain FTs were good pullers, too.
As far as brands, the closest I have to the same locomotive from different manufacturers are Kato RS2s and Atlas RSD4/5s. The Katos pull about 14 up the helix, the Atlases about 12.
Given those numbers, I wouldn’t expect any more than 5 or 6 up a 4.25-percent grade from anything but the best of pullers.