Regarding your question to Mark concerning grades over on his page .... it's all a matter of how many cars you want to pull and how heavy they are. I prefer freights of 15ish cars, as that's the capacity of my arrival/departure tracks.
I just became able to run trains on my layout-under-construction in April. It has 2 grade segments in the 3-3½% range, depending on specific location and variations inevitable in construction over a slightly-sloped floor. Both have curves (104 & 112 diameter). Track is GG & Ross.
So far, engines lacking traction tires are the most-challenged. A Lionel 6-18064 Mohawk is limited to about 8 cars in order to avoid slipping when going uphill. I have only two other such engines, the 6-18005 Hudson and the 6-18009 Mohawk, which I haven't tried yet.
In contrast, the newer 6-35083 L2a Mohawk, with tires, hauls 15 cars with aplomb, as does my 6-14553 scale F3 and a 1990s Weaver Baldwin Shark. My 3rd Rail Dreyfuss Hudson handled 6 K-Line 18" aluminum cars without issue, but I suspect that longer trains may prove challenging for it.